A&A 616, A19 (2018) Astronomy https://doi.org/10.1051/0004-6361/201732518 & © ESO 2018 Astrophysics

The of disks around T Tauri and Herbig Ae/Be stars Marcelino Agúndez1, Evelyne Roueff2, Franck Le Petit2, and Jacques Le Bourlot2,3

1 Instituto de Física Fundamental, CSIC, C/ Serrano 123, 28006 Madrid, Spain e-mail: [email protected] 2 Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 92190 Meudon, France 3 Université Paris-Diderot, Sorbonne Paris-Cité, 75013 Paris, France

Received 21 December 2017 / Accepted 25 March 2018

ABSTRACT

Context. Infrared and (sub-)millimeter observations of disks around T Tauri and Herbig Ae/Be stars point to a chemical differentiation, with a lower detection rate of in disks around hotter stars. Aims. We aim to investigate the underlying causes of the chemical differentiation indicated by observations and perform a comparative study of the chemistry of T Tauri and Herbig Ae/Be disks. This is one of the first studies to compare the chemistry in the outer regions of these two types of disk. Methods. We developed a model to compute the of a generic protoplanetary disk, with particular attention to the , and applied it to a T Tauri and a Herbig Ae/Be disk. We compiled cross sections and computed photodissociation and photoionization rates at each location in the disk by solving the far- (FUV) radiative transfer in a 1+1D approach using the Meudon PDR code and adopting observed stellar spectra. Results. The warmer disk temperatures and higher ultraviolet flux of Herbig stars compared to T Tauri stars induce some differences in the disk chemistry. In the hot inner regions, H2O, and simple organic molecules like C2H2, HCN, and CH4 are predicted to be very abundant in T Tauri disks and even more in Herbig Ae/Be disks, in contrast with infrared observations that find a much lower detection rate of and simple organics toward disks around hotter stars. In the outer regions, the model indicates that the molecules typically + observed in disks, like HCN, CN, C2H, H2CO, CS, SO, and HCO , do not have drastic abundance differences between T Tauri and Herbig Ae disks. Some species produced under the action of photochemistry, like C2H and CN, are predicted to have slightly lower abundances around Herbig Ae stars due to a narrowing of the photochemically active layer. Observations indeed suggest that these radicals are somewhat less abundant in Herbig Ae disks, although in any case, the inferred abundance differences are small, of a factor of a few at most. A clear chemical differentiation between both types of disks concerns ices. Owing to the warmer temperatures of Herbig Ae disks, one expects snow lines lying farther away from the star and a lower mass of ices compared to T Tauri disks. Conclusions. The global chemical behavior of T Tauri and Herbig Ae/Be disks is quite similar. The main differences are driven by the warmer temperatures of the latter, which result in a larger reservoir or water and simple organics in the inner regions and a lower mass of ices in the outer disk. Key words. – molecular processes – protoplanetary disks – stars: variables: T Tauri, Herbig Ae/Be

1. Introduction (Dutrey et al. 1997; Kastner et al. 1997, 2014; van Zadelhoff et al. 2001; Thi et al. 2004; Fuente et al. 2010; Guilloteau et al. Circumstellar disks around young stars, the so-called protoplane- 2013, 2016; Pacheco-Vázquez et al. 2015). Interferometers that tary disks, are an important link in the evolution from molecular operate at (sub-)millimeter wavelengths such as the OVRO mil- clouds to planetary systems. These disks allow feeding of the limeter array, the IRAM array at Plateau de Bure (PdBI, now young star with matter and provide the scenario in which plan- known as NOEMA), and the Submillimeter Array (SMA) have ets form. The study of the physical and chemical conditions of also permitted sensitive observations leading to the detection of + these objects is thus of paramount importance to understanding new molecules such as N2H and HC3N, and to obtain maps of how planets form and the types of material incorporated. Proto- the emission distribution of some molecules with angular reso- planetary disks are mainly composed of molecular gas and dust. lutions down to a few arcseconds (Qi et al. 2003, 2008, 2013a; The last two decades have seen significant progress in the study Dutrey et al. 2007, 2011; Piétu et al. 2007; Schreyer et al. 2008; of their chemical composition thanks to astronomical observa- Henning et al. 2010; Öberg et al. 2010, 2011; Chapillon et al. tions at wavelengths from the millimeter to the ultraviolet (UV) 2012a,b; Fuente et al. 2012; Graninger et al. 2015; Teague et al. domains. 2015; Pacheco-Vázquez et al. 2016). In recent years, the advent Observations with ground-based millimeter and sub- of the Atacama Large Millimeter Array (ALMA) has made it millimeter telescopes such as the 30 m antenna of the Institut possible to characterize the molecular content of protoplane- de Radioastronomie Millimétrique (IRAM), the James Clerk tary disks with unprecedented sensitivity and angular resolution Maxwell Telescope (JCMT), and the APEX 12 m telescope, (down to sub-arcsecond scales). For example, thanks to ALMA, which are sensitive to the outer cool regions of disks, have pro- it has been possible to detect new molecules such as cyclic C3H2 vided information on the presence of various gaseous molecules (Qi et al. 2013b; Bergin et al. 2016), CH3CN (Öberg et al. 2015), + such as CO, HCO ,H2CO, C2H, HCN, HNC, CN, CS, and SO and CH3OH (Walsh et al. 2016), and to image the CO snowline

Article published by EDP Sciences A19, page 1 of 33 A&A 616, A19 (2018) in a few disks (Mathews et al. 2013; Qi et al. 2013c, 2015; (Woods & Willacy 2007) and complex organic molecules (Walsh Schwarz et al. 2016; Zhang et al. 2017). Infrared (IR) observa- et al. 2014). tions using space telescopes such as Spitzer and ground-based Overall, protoplanetary disks are complex systems where facilities, such as the Very Large Telescope (VLT) and the Keck many different processes govern the chemical composition: Observatory telescopes have provided a view of the molecu- for example, gas phase chemistry, interaction with stellar lar content of the very inner regions of protoplanetary disks, and interstellar FUV photons, transport processes, adsorption, where absorption and emission lines from molecules, such as and desorption from dust grains, chemical reactions on grain CO, CO2,H2O, OH, HCN, C2H2, and CH4 have been routinely surfaces, and grain evolution (see reviews by Bergin et al. 2007; observed (Lahuis et al. 2006; Gibb et al. 2007; Salyk et al. 2007, Henning & Semenov 2013; Dutrey et al. 2014; Pontoppidan et al. 2008, 2011; Carr & Najita 2008, 2011, 2014; Pontoppidan et al. 2014). 2010a,b; Najita et al. 2010, 2013; Kruger et al. 2011; Doppmann Disks are commonly found around young low-mass (T Tauri) et al. 2011; Fedele et al. 2011; Mandell et al. 2012; Bast et al. and intermediate-mass (Herbig Ae/Be) stars, which have quite 2013; Gibb & Horne 2013; Sargent et al. 2014; Banzatti et al. different masses and effective temperatures, and thus can 2017). The launch of the Herschel Space Observatory was also affect the disk chemical composition differently. For example, very helpful to investigate the chemical content at far-IR wave- Herbig Ae/Be stars have a higher UV flux and disks around lengths, with the detection of molecules, such as CH+ (Thi et al. them are warmer than around T Tauri stars. Indeed, T Tauri and 2011) and NH3 (Salinas et al. 2016), and the exhaustive charac- Herbig Ae disks have been extensively observed from millime- terization of H2O and OH from the inner to the outer regions ter to IR wavelengths and it has been found that the detection (Hogerheijde et al. 2011; Riviere-Marichalar et al. 2012; Meeus rate of molecules (e.g., H2O, C2H2, HCN, CH4, CO2,H2CO, + et al. 2012; Fedele et al. 2012, 2013; Podio et al. 2013). Probing C2H, and N2H ) is strikingly lower toward Herbig Ae disks the molecular gas in the inner regions of disks through the most than toward disks around T Tauri stars (Mandell et al. 2008; abundant , H2, has also been possible thanks to obser- Schreyer et al. 2008; Pontoppidan et al. 2010a; Öberg et al. vations at UV wavelengths using the Hubble Space Telescope 2010, 2011; Fedele et al. 2011, 2012, 2013; Salyk et al. 2011; (Ingleby et al. 2009; France et al. 2012). Riviere-Marichalar et al. 2012; Meeus et al. 2012; Guilloteau On the theoretical side, the chemical structure of protoplan- et al. 2016; Banzatti et al. 2017). This fact may indicate that there etary disks has also been widely studied during the last two is a marked chemical differentiation between these two types of decades. Early models focused on the one dimensional radial disk. Most theoretical studies on the chemistry of protoplanetary structure of disks along the midplane (Aikawa et al. 1996, 1997, disks have focused on disks around T Tauri-like stars, and only 1999; Willacy et al. 1998; Aikawa & Herbst 1999a), although a few have studied Herbig Ae/Be disks (e.g., Jonkheid et al. it was later recognized that the chemical composition also 2007). Here we present a comparative study in which we inves- presents an important stratification along the vertical direction, tigate the two-dimensional (2D) distribution of molecules in with a structure consisting of three main layers: the cold mid- disks around T Tauri and Herbig Ae/Be stars. In a recent study, plane where molecules are mostly condensed as ices on dust Walsh et al.(2015) investigated the differences in the chemi- grains, a warm upper layer where a rich chemistry takes place, cal composition between disks around stars of different spectral and the uppermost surface layer where photochemistry driven type, focusing on the inner disk regions. In the present study, by stellar and interstellar far-ultraviolet (FUV) photons regu- we make a thorough investigation of the main chemical differ- lates the chemical composition (Aikawa & Herbst 1999b, 2001; ences and similarities between T Tauri and Herbig Ae/Be disks Willacy & Langer 2000; Aikawa et al. 2002). In recent years from the inner to the outer disk regions, which to our knowl- there has been an interest in identifying the main processes that edge has not been investigated in detail. We are particularly affect the abundance and distribution of molecules in proto- concerned with a detailed treatment of the photochemistry and planetary disks. A large number of studies have addressed in with the impact of the FUV illumination from these two types detail the role of processes, such as the interaction with FUV of stars on the chemical composition of the disk. To this pur- and X-ray radiation (Willacy & Langer 2000; Markwick et al. pose we have implemented the Meudon PDR code in the disk 2002; Bergin et al. 2003; Ilgner & Nelson 2006a; Agúndez et al. model to compute photodestruction rates at each location in 2008; Walsh et al. 2010, 2012; Aresu et al. 2011; Fogel et al. the disk in a 1+1D approach. We adopted FUV stellar spectra 2011), the interplay between the thermal and chemical structure coming from observations of representative T Tauri and Her- (Glassgold et al. 2004; Woitke et al. 2009), turbulent mixing big Ae/Be stars. In Sect.2 we present in detail our physical and and other transport processes (Ilgner et al. 2004; Semenov et al. chemical disk model, with particular emphasis on the photo- 2006, 2011; Ilgner & Nelson 2006b; Willacy et al. 2006; Aikawa chemistry (further described in AppendicesA andB). In Sect.3 2007; Heinzeller et al. 2011), and the evolution of dust grains as we present the resulting abundance distributions of important they grow by coagulation and settle onto the midplane regions families of molecules for our T Tauri and Herbig Ae/Be disk (Aikawa & Nomura 2006; Fogel et al. 2011; Akimkin et al. models and compare them with results from observations. In 2013). Some studies have investigated the impact of using dif- Sect.4 we analyze the influence on the chemistry of the stellar ferent chemical networks (Semenov et al. 2004; Ilgner & Nelson spectra and the method used to compute photodestruction rates, 2006c; Kamp et al. 2017) and the sensitivity to uncertainties in and we summarize the main conclusions found in this work in the rate constants of chemical reactions (Vasyunin et al. 2008). Sect.5. Various specific aspects of the chemistry of protoplanetary disks have also been addressed in detail, such as fraction- 2. The disk model ation (Willacy 2007; Willacy & Woods 2009; Thi et al. 2010; Furuya et al. 2013; Yang et al. 2013; Albertsson et al. 2014), We consider a passively irradiated disk in steady state around a the formation and survival of water vapor (Dominik et al. 2005; T Tauri or Herbig Ae/Be star. We solve the thermal and chemical Glassgold et al. 2009; Bethell & Bergin 2009; Du & Bergin structure of the disk using a procedure which can be summa- 2014), and the formation of particular species such as rized as follows. We first solve the dust temperature distribution

A19, page 2 of 33 M. Agúndez et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars

Table 1. Model parameters. Draine & Lee 1984; Laor & Draine 1993; Weingartner & Draine 2001), and the size distribution being given by the power law Parameter Value/Source β n(a) a− , (2) Disk ∝ Disk mass (Mdisk) 0.01 M n a a Inner disk radius (Rin) 0.5 au where ( ) is the number of grains of radius , the minimum and Outer disk radius (Rout) 500 au maximum grain radius amin and amax are 0.001 and 1 µm, and the Radial surface density power index () 1.0 exponent β takes a value of 3.5 according to Mathis et al.(1977). † Gas-to-dust mass ratio (ρg/ρd) 100 As stellar parameters we adopt typical values of T Tauri and Dust composition 70% silicate Herbig Ae/Be stars. The T Tauri star is assumed to have a mass 30% graphite M of 0.5 M , a radius R of 2 R , and an effective temperature ∗ ∗ Minimum dust grain radius (amin) 0.001 µm T of 4000 K, typical values of T Tauri stars of spectral type Maximum dust grain radius (amax) 1 µm M0-K7∗ ∼ in Taurus (Kenyon & Hartmann 1995). In the case of the Dust size distribution power index (β) 3.5 star of type Herbig Ae/Be we adopt a mass of 2.5 M , a radius of Interstellar radiation field Draine(1978) 17 1 2.5 R , and an effective temperature of 10 000 K, typical val- Cosmic-ray ionization rate of H (ζ) 5 10− s− 2 × ues of early Ae and late Be stars (e.g., Martin-Zaïdi∼ et al. 2008; T Tauri star Montesinos et al. 2009). Stellar mass (M ) 0.5 M Stellar radius (R∗ ) 2 R Stellar effective∗ temperature (T ) 4000 K 2.1.1. Stellar and interstellar FUV spectra ∗ Herbig Ae/Be star The irradiation from the central star is of great importance for Stellar mass (M ) 2.5 M the disk because it dominates the heating of gas and dust. The Stellar radius (R∗ ) 2.5 R ∗ stellar and interstellar spectra at FUV wavelengths are also of Stellar effective temperature (T ) 10 000 K ∗ prime importance because they control the photochemistry that takes place in the disk surface. Notes. † = 12 at r < Rin. − The FUV component of the interstellar radiation field (ISRF) in the disk using the RADMC code (Dullemond & Dominik 2004). adopted here is given by We assume that gas and dust are thermally coupled except for the surface layers of the disk where we estimate the gas kinetic 1 6.3622 107 1.0238 1011 4.0813 1013 Iλ = × × + × , (3) temperature following Kamp & Dullemond(2004). We then 4π λ4 − λ5 λ6 solve the radiative transfer of interstellar FUV photons along the   vertical direction and of stellar FUV photons along the direc- where λ is the wavelength in Å, Iλ is the specific intensity in 1 2 1 1 tion from the star using the Meudon PDR code (Le Petit et al. units of erg s− cm− Å− sr− , and the expression has been 2006) to get the photodissociation and photoionization rates of obtained by fitting to the radiation field given by Draine(1978). the different species at each disk location. We finally solve the The expression in Eq. (3) is similar to that given by Le Petit chemical composition at each location in the disk as a func- et al.(2006), except for an erratum in their first term. The ISRF tion of time including gas phase chemical reactions, processes given by Eq. (3) is complemented with another component that induced by FUV photons and cosmic rays, and interactions accounts for the emission at longer wavelengths (from 2000 Å of gas particles with dust grains (adsorption and desorption to the near IR), for which we adopt the radiation field∼ given by processes). Mathis et al.(1983) in the form of a combination of three diluted black bodies 2.1. Physical model 14 13 I = 1.05 10− B (7127K) + 1.28 10− B (4043K) λ × λ × λ We adopt a fiducial model of disk representative of objects 13 + 3.30 10− B (2930K), (4) commonly found around T Tauri and Herbig Ae/Be stars (see × λ parameters in Table1). The disk extends between an inner radius where Bλ(T) is the Planck law at temperature T. Rin of 0.5 au and an outer radius Rout of 500 au from the star and Concerning stellar spectra, in the case of the T Tauri star we has a mass Mdisk of 0.01 M . We consider that the radial distri- bution of the surface density Σ is given by a power law of the adopt as a proxy of the stellar spectrum that of TW Hya, which type consists of observations with the Far Ultraviolet Spectroscopic Explorer (FUSE) in the 900–1150 Å wavelength range (data  Σ = Σ0(r/r0)− , (1) from program C0670102) and Hubble STIS observations in the 1150–3150 Å range (Herczeg et al. 2002; Bergin et al. where Σ0 is the surface density at a reference radius r0 and the 2003). The intrinsic stellar brightness is calculated adopting a exponent  is chosen to be 1.0 between Rin and Rout. To avoid the distance to the star of 51 pc (Mamajek 2005), a stellar radius of abrupt disappearance of the disk at the inner radius we set  to 1 R (Webb et al. 1999), and a negligible interstellar reddening

12 at r < Rin, which results in a soft continuation of the disk at (Bergin et al. 2003). At wavelengths longer than 3150 Å we − the inner regions. use a Kurucz model spectrum (Castelli & Kurucz 2004)1 We consider that dust is present in the disk with a uniform with an effective temperature of 4000 K, a surface gravity of 1.5 2 abundance and size distribution, i.e., they do not vary with radius 10 cm s− , and solar , scaled to match the flux of or height over midplane. We adopt a gas-to-dust mass ratio of 100 TW Hya around 3150 Å. The resulting spectrum is similar to that and consider spherical grains typically present in the interstellar presented by France et al.(2014) in the 1150–1750 Å wavelength medium, that is, with the composition being a mixture of 70% of silicate and 30% of graphite (with optical properties taken from 1 http://wwwuser.oats.inaf.it/castelli

A19, page 3 of 33 Agundez´ et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars A&A 616, A19 (2018)

Table 1. Model parameters. 5 dust temperature as a function of radius r and height z over the 10 FUV flux at 1 au 10,000 K blackbody midplane. The vertical distribution of the volume density of par- Parameter Value / Source 104 ticles n(z) is assumed to be given by hydrostatic equilibrium as

Disk ) Herbig Ae/Be (AB Aurigae) 1 Disk mass (Mdisk) 0.01 M − 3 Å dn(z) µGM z 10 1 = ∗ dz, (5) Inner disk radius (Rin) 0.5 au − 2 2 3/2 s n(z) −kT (z) (z + r ) d

Outer disk radius (Rout) 500 au 2 2

− 10 

Radial surface density power index ( ) 1.0 m T Tauri (TW Hya)

† c where µ is the mean mass of particles, G the gravitational Gas-to-dust mass ratio (ρg/ρd) 100 1 g 10 r constant, k the Boltzmann constant, and Td(z) is the vertical dis- Dust composition 70 % silicate e (

λ tribution of dust temperature. We proceed iteratively to find n(z) 30 % graphite 0 F 10 4000 K blackbody Minimum dust grain radius (amin) 0.001 µm at each radius r according to Eq. (5) and consistently with the a µ -1 vertical distribution of dust temperature Td(z) computed at each Maximum dust grain radius ( max) 1 m 10 Dust size distribution power index (β) 3.5 radius r. Interstellar radiation field Draine (1978) 1000 1500 2000 2500 3000 We assume that gas and dust are thermally coupled, that is, 17 1 Cosmic-ray ionization rate of H (ζ) 5 10− s− 2 × λ (Å) that gas and dust temperatures are equal, except for the disk T Tauri star surface. Models dealing with the computation of the gas temper- Stellar mass (M ) 0.5 M Stellar FUV flux at 1 au of the T Tauri star (emitting as TW Hya ∗ Fig. 1. Stellar FUV flux at 1 au of the T Tauri star (emitting as TW Hya ature in protoplanetary disks find that the thermal coupling of Stellar radius (R ) 2 R and as a 4000 K blackbody) and of the Herbig Ae/Be star (emitting as ∗ and as a 4000 K blackbody) and of the Herbig Ae/Be star (emitting as gas and dust is a good approximation over most of the disk and Stellar effective temperature (T ) 4000 K AB Aurigae and as a 10,000 K blackbody). ∗ AB Aurigae and as a 10 000 K blackbody). that this assumption breaks down at the surface layers of the disk, Herbig Ae/Be star A Stellar mass (M ) 2.5 M where the visual extinction V in the vertical outward direction ∗ becomes lower than 1 (Kamp & Dullemond 2004; Woitke et al. Stellar radius (R ) 2.5 R whererange,n although(a) is the their number continuum of grains level of radius is abouta, the twice minimum below and our ∼ Stellar effective∗ temperature (T ) 10,000 K adopted spectrum. In thea case ofa the star of type Herbigµ Ae/Be 2009; Walsh et al. 2010). In these surface layers the gas can be ∗ maximum grain radius min and max are 0.001 and 1 m, and the  = 12 at r < Rin. β much warmer than dust grains. In order to take this into account † − exponentwe use astakes a proxy a value the of FUV 3.5 accordingspectrum toof Mathis AB Aurigae, et al. (1977). which consistsAs stellar of observationsparameters we taken adopt typical with FUSE values of in T theTauri 900– and we use the following approximation for the gas temperature. We Herbig1190 Å Aewavelength/Be stars. The range T Tauri (data star from is programassumed toP2190301) have a mass and assume that the gas temperature is equal to the dust tempera- ture in those regions where A in the vertical outward direction perature following Kamp & Dullemond (2004). We then solve withM ofHubble 0.5 M STIS, a radius in theR 1190–1710of 2 R ,Å andrange an e (ffRobergeective temperature et al. 2001; V ∗ ∗ is higher than 1. Following the study by Kamp & Dullemond the radiative transfer of interstellar FUV photons along the ver- TAyresof 20104000). K, The typical intrinsic values brightness of T Tauri of ABstars Aurigae of spectral is calcu- type ∗ ∼ (2004), we assume that in the uppermost regions of the disk, tical direction and of stellar FUV photons along the direction M0-K7lated adopting in Taurus a distance (Kenyon to & the Hartmann star of 144 1995). pc, aIn stellar the case radius of the of where A < 0.01, the gas temperature is equal to the evaporation from the star using the Meudon PDR code (Le Petit et al. 2006) star2.41 ofR type, and Herbig an interstellar Ae/Be we extinction adopt a mass of 0.48 of 2.5 mag M (Martin-Zaïdi, a radius of V temperature of a , calculated as the temperature to get the photodissociation and photoionization rates of the dif- 2.5et al. R 2008, and). an For effective the correction temperature due of to extinction10,000 K, typical we use val- the ∼ at which the most probable speed of particles equals the escape ferent species at each disk location. We finally solve the chemi- uesmethod of early and Ae coefficients and late Be of starsFitzpatrick (e.g., Martin-Za & Massa(¨ıdi2007 et). al. Long- 2008; velocity from the disk, that is, cal composition at each location in the disk as a function of time Montesinoswards of 1710 et al.Å we 2009). use a Kurucz spectrum (Castelli & Kurucz including gas phase chemical reactions, processes induced by 2004) with an effective temperature of 9750 K (close to that GM mH FUV photons and cosmic rays, and interactions of gas particles 2.0 2 Tevap = ∗ , (6) 2.1.1.of AB Stellar Aurigae), and a interstellar surface gravity FUV spectra of 10 cm s− , and solar kr with dust grains (adsorption and desorption processes). metallicity. TheThe irradiation FUV spectra from the adopted central for star the is T Tauriof great and importance Herbig Ae/Be for where mH is the mass of a hydrogen atom. Finally, at regions thestars disk are because shown in it Fig. dominates1, where the we heating also compare of gas and with dust. spectra The intermediate between AV = 1 and AV = 0.01 we approximate 2.1. Physical model stellarcorresponding and interstellar to blackbodies spectra at at the FUV temperatures wavelengths of are the alsoT Tauri of the gas temperature through a linear interpolation with height. We adopt a fiducial model of disk representative of objects com- primeand Herbig importance Ae/Be because stars, 4000 they and control 10 000 the K, photochemistry respectively. thatIt is In Fig.2 we show the 2D distributions of the volume density monly found around T Tauri and Herbig Ae/Be stars (see param- takesseen that place AB in Aurigae the disk outshines surface. the FUV flux of TW Hya by 2–3 of particles and of the gas and dust temperatures calculated for eters in Table 1). The disk extends between an inner radius R ordersThe of FUV magnitude component due of to the the interstellar much higher radiation effective field temper- (ISRF) the disks around the T Tauri and Herbig Ae/Be stars. The flared in adopted here is given by of 0.5 au and an outer radius Rout of 500 au from the star and has ature. However, T Tauri stars usually have an important FUV shape, which is apparent in both disks, is more prominent in the excess and may become very bright in lines such as Lyα (at T Tauri disk owing to the lower gravity of the star. The most sig- a mass Mdisk of 0.01 M . We consider that the radial distribution 1 6.3622 107 1.0238 1011 4.0813 1013 Σ 1215.67= Å). It is worth noting that TW+ Hya is brighter, than nificant difference between the physical structure of both disks of the surface density is given by a power law of the type Iλ 4× × × (3) AB Aurigae4π inλ the Lyα−line andλ5 that in TW Hyaλ the6 fraction of is that the disk around the Herbig Ae/Be star is significantly    flux emitted in Lyα is about 30% of the total flux emitted in the warmer than the disk around the T Tauri star because of the Σ = Σ0(r/r0)− , (1) where λ is the wavelength in Å, Iλ is the specific intensity in 910–2400 Å wavelength1 2 1 range.1 We also note that while the FUV higher stellar irradiation of the former. units of erg s− cm− Å− sr− , and the expression has been ob- where Σ0 is the surface density at a reference radius r0 and the tainedspectra by of ABfitting Aurigae to the is radiation similar to field that given of a 10 by 000 Draine K blackbody, (1978). exponent  is chosen to be 1.0 between Rin and Rout. To avoid the Thea 4000 expression K blackbody in Eq. is (3) a badis similar approximation to that given for by a T Le Tauri Petit star et al. as 2.2. Chemical model abrupt disappearance of the disk at the inner radius we set  to it completely misses the FUV excess. As is discussed in Sect.4, (2006), except for an erratum in their first term. The ISRF given Once the physical structure of the disk (temperature of gas and 12 at r < Rin, which results in a soft continuation of the disk at bythis Eq. has (3) important is complemented consequences with for another the chemistry component of the that disk. ac- −the inner regions. dust and volume density of particles) is calculated at steady state, counts for the emission at longer wavelengths (from 2000 Å we solve for the temporal evolution of the chemical composi- We consider that dust is present in the disk with a uniform to2.1.2. theDust near IR), and forgas which temperature we adopt the radiation field∼ given by abundance and size distribution, i.e., they do not vary with ra- tion at each location in the disk. Since transport processes are Mathis et al. (1983) in the form of a combination of three diluted neglected, each location in the disk evolves independently of dius or height over midplane. We adopt a gas-to-dust mass ra- blackGiven bodies the input parameters characteristic of the star (M , R , and tio of 100 and consider spherical grains typically present in stellar spectrum), the radial distribution of surface density∗ ∗ in the other disk regions. We solve the chemical composition as a func- 14 13 tion of time up to 1 Myr, which is of the order of the typical the , that is, with the composition being a diskI = 1 given.05 by10− Eq.B (1(7127K)), the dust-to-gas+1.28 10 massB ratio,(4043K) and the dust λ λ − λ ages of protoplanetary disks, for a grid consisting of 50 radii mixture of 70 % of silicate and 30 % of graphite (with optical properties,× we solve for the 2D distribution× 13 of the dust tempera- +3.30 10− Bλ(2930K), (4) properties taken from Draine & Lee 1984, Laor & Draine 1993, ture in the disk using the RADMC code× (Dullemond & Dominik (logarithmically spaced from Rin to Rout) and 200 heights (gener- 2 ated specifically for each radius to properly sample the different and Weingartner & Draine 2001), and the size distribution being where2004) B. λRADMC(T) is theis aPlanck 2D Monte law at Carlo temperature code thatT. solves the dust given by the power law continuumConcerning radiative stellar transfer spectra, in circumstellarin the case of disks the T and Tauri yields starwe the regimes of AV in the vertical direction). The chemical net- adopt as a proxy of the stellar spectrum that of TW Hya, which work includes 252 species (97 neutral species, 133 positive β 2 http://www.ita.uni-heidelberg.de/~dullemond/ plus the negative H− and free electrons, and 20 ice n(a) a− , (2) consists of observations with the Far Ultraviolet Spectroscopic ∝ software/radmc/ molecules) involving the elements H, He, C, N, O, S, Cl, and F.

A19, page 4 of 33 3 M.Ag Agúndezundez´ et et al.: al.: The The chemistry chemistry of of disks disks around around T T Tauri Tauri and and Herbig Herbig Ae Ae/Be/Be stars stars

1.0 1014 1.0 1014 T Tauri 1013 Herbig Ae/Be 1013 12 12 3 10 3 10 0.8 H nuclei density (cm− ) 0.8 H nuclei density (cm− ) 1011 1011 1010 1010 0.6 109 0.6 109

r 8 r 8 / 10 / 10 z 7 z 7 0.4 10 0.4 10 106 106 105 105 0.2 4 0.2 4 10 10 103 103 0.0 102 0.0 102 100 101 102 100 101 102

1.0 1.0 T Tauri Herbig Ae/Be 0.8 gas temperature (K) 0.8 gas temperature (K) 103 103 0.6 0.6 r r / / z z 0.4 0.4 102 102 0.2 0.2

0.0 0.0 100 101 102 100 101 102

1.0 1.0 T Tauri Herbig Ae/Be 0.8 dust temperature (K) 0.8 dust temperature (K) 103 103 0.6 0.6 r r / / z z 0.4 0.4 102 102 0.2 0.2

0.0 0.0 100 101 102 100 101 102 r (au) r (au)

Fig. 2. Calculated volume density of H nuclei (top panel), gas temperature (middle panel), and dust temperature (bottom panel) as a function of radiusFig. 2.rCalculatedand height volume over radius densityz/r for of H the nuclei T Tauri (top), (left gas panel temperature) and Herbig (middle), Ae/Be and (right dust panel temperature) disks. The (bottom) dashed as lines a function indicate of theradius locationr and where height AoverV in the radius outwardz/r for vertical the T direction Tauri (left) takes and values Herbig of Ae 0.01/Be and (right) 1. disks. The dashed lines indicate the location where AV in the outward vertical direction takes values of 0.01 and 1.

Atoms of Si, P, Fe, Na, and Mg are also included because their processes between the gas and ice mantle phases (adsorption ionizedmetal”case forms (e.g., are Graedel important et al. in 1982; controlling Lee et the al. 1998), degree are of listed ion- and2015). desorption). Therefore, At it will this be stage, interesting the model to study does the notimpact include of X izationin Table in 3 certain of Agundez´ disk regions. & Wakelam We adopt(2013). an The initial chemical chemical net- X-ray-inducedrays on the chemistry processes of the and two grain-surface types of disks reactions, in the future. except In work comprises 5533 processes including gas phase chemical any case, previous chemical models of T Tauri disks have found composition calculated with a pseudo-time-dependent chemi- for the formation of H2. X-rays may be an important source calreactions, model (where cosmic-ray the chemicalinduced processes, evolution photodissociations is solved under fixed and ofthat chemical the gas-phase differentiation chemistry between is not greatly disks around affected T by Tauri X rays. and photoionizations due to stellar and interstellar FUV photons, and The species whose abundance is most affected are, according physical conditions) of a cold dense cloud with standard param- Herbig Ae/Be stars because the former are more important+ exchange processes between the gas and4 ice3 mantle phases (ad- to Aresu et al. (2011), the ions present in the disk surface OH , eters (density of H nuclei of 2 10 cm− , temperature of X-ray+ emitters+ (see, e.g.,+ Telleschi et al. 2007). Moreover, winds+ 10sorption K, visual and extinction desorption). of At 10 this mag,× stage, and the cosmic-ray model does ionization not in- andH2O magnetic,H3O , fields and N in, while actively Walsh accreting et al. (2012) T Tauri find systems that N may2H clude X-ray-induced17 processes1 and grain-surface reactions, ex- is the most sensitive species to X rays. We however note that rate of H2 of 5 10− s− ) at a time of 0.1 Myr. The elemen- lead to cosmic-ray exclusion meaning that ionization in the disk talcept abundances for the formation adopted,× of based H2. X on rays the may so-called be an “low important metal” source case canthe models be dominated of Aresu by et X-rays al. (2011) rather and than Walsh cosmic et al. (2012)rays (Cleeves did not (e.g.,of chemical Graedel di etff al.erentiation 1982; Lee between et al. 1998 disks), are around listed T in Tauri Table and 3 etconsider al. 2015 cosmic-ray). Therefore, exclusion, it will be interestingunlike the study to study of theCleeves impact et ofal. ofHerbig Agúndez Ae/Be & Wakelamstars because 2013 the. The former chemical are more network important comprises X-ray X-rays(2015). on Chemical the chemistry reactions of the occurring two types of on disks the surfacein the future. of dust In 5533emitters processes (see, e.g., including Telleschi gas phase et al. 2007). chemical Moreover, reactions, winds cosmic- and anygrains case, are previous likely to chemical have an models effect on of the T Tauri chemical disks composition have found raymagnetic induced fields processes, in actively photodissociations accreting T Tauri and systems photoionizations may lead thatof cool the midplane gas-phase regions, chemistry especially is not greatly regarding affected complex by X-rays.organic dueto cosmic-ray to stellar exclusion and interstellar meaning FUV that ionization photons, and in the exchange disk can Themolecules species (Walsh whose et abundance al. 2014), is although most affected the distribution are, according of abun- to be dominated by X rays rather than cosmic rays (Cleeves et al. dant molecules and the main chemical patterns in the disk are

A19, page 5 of 33 5 A&A 616, A19 (2018)

Aresu et al.(2011), the ions present in the disk surface OH +, et al. 2013) and KIDA (Wakelam et al. 2012, 2015), compi- + + + + H2O ,H3O , and N , while Walsh et al.(2012) find that N 2H lations for application in , such as the is the most sensitive species to X-rays. We however note that evaluations by IUPAC (Atkinson et al. 2004, 2006)7 and JPL the models of Aresu et al.(2011) and Walsh et al.(2012) did (Sander et al. 2011)8, and compilations for use in combustion not consider cosmic-ray exclusion, unlike the study of Cleeves chemistry, such as the IUPAC evaluation by Baulch et al.(2005) et al.(2015). Chemical reactions occurring on the surface of dust and the Leeds oxidation mechanism (Hughes et al. grains are likely to have an effect on the chemical composition 2001) or the mechanism by Konnov(2000). A good number of of cool midplane regions, especially regarding complex rate constants have been taken from specific experimen- molecules (Walsh et al. 2014), although the distribution of abun- tal and theoretical studies found in the literature on chemical dant molecules and the main chemical patterns in the disk are kinetics. For example, we have included the numerous measure- probably not greatly affected by such processes. We also plan to ments at low and ultra-low temperatures carried out with the investigate this particular aspect in the future. CRESU apparatus (Smith et al. 2006). It is important to note that some regions of protoplanetary disks may have tempera- tures up to some thousands of degrees Kelvin and therefore 2.2.1. Gas phase chemical reactions it is necessary to include reactions that become fast at high The vast majority of gas-phase chemical reactions included can temperatures, i.e., reactions which are endothermic and/or have be grouped into two main categories: ion neutral reactions and activation barriers. data for such reactions neutral neutral reactions. The subset of− ion neutral reactions are to a large extent based on chemical networks used in pre- has been− constructed based on databases originally− devoted to vious chemical models of warm gas in protoplanetary nebulae the study of the chemistry of cold interstellar clouds, such as the (Cernicharo 2004) and inner regions of protoplanetary disks UMIST database for astrochemistry (Woodall et al. 2007; McElroy (Agúndez et al. 2008), whose original sources of data are mainly et al. 2013)3 and the Ohio State University (OSU) database, the combustion chemistry databases listed above. A similar high- formerly maintained by E. Herbst and currently integrated into temperature chemical network was also used by Harada et al. the Kinetic Database for Astrochemistry (KIDA; Wakelam et al. (2010) to model the chemistry of active galactic nuclei (AGNs). 2012, 2015)4. Rate constants of ion neutral reactions have been We also include three-body reactions and their reverse pro- − taken from the previous databases and from the literature on cess (thermal dissociation) with H2, He, and H acting as third chemical kinetics. In particular, a large part of the rate constants body. Three-body reactions become important at densities above 10 3 has been revised according to the compilation by Anicich 10 cm− , values that are reached in the innermost midplane (2003)5. The chemical kinetics of exothermic ion neutral regions∼ of protoplanetary disks, while thermal dissociations reactions is rather simple because in most cases the kinetics− is become important at high temperatures. We use an expanded ver- dominated by long range electrostatic forces. In the case of reac- sion of the set of three-body reactions and thermal dissociations tions in which the neutral species is nonpolar the theory indicates compiled by Agúndez & Cernicharo(2006). An important aspect that the rate constant is independent of temperature and is given of the neutral–neutral subset of reactions is that for many of the by the Langevin value. If the neutral species has an electric endothermic reactions for which chemical kinetics data are not dipole moment the expression found by Su & Chesnavich(1982) available the rate constants have been calculated through detailed can be used to evaluate the rate constant and its dependence with balance from the rate constant of the reverse exothermic reaction temperature (Maergoiz et al. 2009; see more details in Wakelam and the thermochemical properties of the species involved. Ther- et al. 2010). For ion–nonpolar reactions for which there is no mochemical data in the form of NASA polynomial coefficients experimental data, the rate constant has been approximated as (McBride et al.(2002) have been obtained from compilations the Langevin value. In the case of ion polar reactions, we have like those by Konnov(2000) and Burcat & Ruscic(2005) 9. used the Su-Chesnavich approach to evaluate− the rate constant of reactions not studied experimentally and to obtain the tempera- 2.2.2. Cosmic-ray-induced processes ture dependence of the rate constant of reactions which has only been characterized at one single temperature, usually around The processes induced by cosmic rays also play an important 300 K. role in the chemistry of protoplanetary disks. We include the The part of the chemical network involving ions also direct ionization of H2 and by cosmic-ray impact, together includes dissociative recombinations of positive ions with with photoprocesses induced by secondary electrons produced in electrons and radiative recombinations between cations and the direct ionization of H2, the so-called Prasad–Tarafdar mech- electrons. The set of reactions and associated rate constants have anism (Prasad & Tarafdar 1983; Gredel et al. 1989). The rates of mainly been taken from databases such as UMIST (Woodall these processes are expressed in terms of the cosmic-ray ioniza- tion rate of H (ζ), for which we adopt a value of 5 10 17 s 1 et al. 2007; McElroy et al. 2013) and KIDA (Wakelam et al. 2 × − − 2012, 2015). Information on the chemical kinetics of dissociative (see Table1), and are taken from astrochemical databases such recombinations has largely benefited from experiments carried as UMIST (Woodall et al. 2007; McElroy et al. 2013) and KIDA out with ion storage rings (Florescu-Mitchell & Mitchell 2006; (Wakelam et al. 2012, 2015). Geppert & Lasson 2008). The subset of neutral–neutral reactions has been constructed 2.2.3. Photoprocesses from chemical kinetics databases, such as the one by NIST Photodissociation and photoionization processes caused by stel- 6 (Manion et al. 2013) , databases devoted to the study of inter- lar and interstellar FUV photons are a key aspect of the chemistry stellar chemistry, such as UMIST (Woodall et al. 2007; McElroy of protoplanetary disks as they control the chemical composi- tion of the surface layers, from where much of the molecular 3 http://udfa.ajmarkwick.net 4 http://kida.obs.u-bordeaux1.fr 7 http://iupac.pole-ether.fr/ 5 http://trs.jpl.nasa.gov/handle/2014/7981 8 http://jpldataeval.jpl.nasa.gov/ 6 http://kinetics.nist.gov 9 http://burcat.technion.ac.il/dir

A19, page 6 of 33 Agundez´ et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars M. Agúndez et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars Sec. 2.1) and prior to the computation of the temporal evolution 0 emission arises. In order to treat these processes in detail 10 CO of the chemical composition, we use the Meudon PDR code to -1 stellar we have used the Meudon PDR code (Le Petit et al. 2006; 10 N2 ) -2 interstellar evaluate the photodissociation and photoionization rates at each 10 1 10 H2O Goicoechea & Le Bourlot 2007; González García et al. 2008) , − OH s -3 location in the disk by calculating the FUV flux as a function of ( 10 where PDR stands for , to compute the CO2 e -4 wavelength due to stellar and interstellar radiation and using the t 10 HCN a

photodissociation and photoionization rates of various important r 10-5 relevant wavelength-dependent cross sections. Our approach to n 1 au 100 au species at each location in the disk. We use version 1.4 of the o -6 i 10 treat photochemistry is thus different from other state-of-the-art t a Meudon PDR code, with some practical modifications to make i 10-7 chemical models of protoplanetary disks in which the FUV ra- c it more versatile and integrate it into the protoplanetary disk o -8 s 10 diative transfer is solved in 2D but only in a few broad spectral s i -9 model. The Meudon PDR code solves the FUV radiative trans- d 10 bands (e.g., Woitke et al. 2016) and it is in essence more similar o

t -10 fer in one dimension for a plane-parallel cloud illuminated on o 10 to the series of models by Walsh et al. (2012, 2014, 2015). h -11 one side by a FUV source. In protoplanetary disks, the geome- p 10 We have compiled cross sections for 29 molecules and 8 10-12 T Tauri disk try involves two dimensions (radial and vertical) and there are atoms (see Appendix A). The photodissociation rate of H2 and two different FUV sources, the star illuminating from the central 0.0 0.2 0.4 0.6 0.8 1.0 CO are computed by solving the excitation and the line-by-line position and the interstellar radiation field illuminating isotrop- 100 radiative transfer taking into account self and mutual shielding stellar CO ically from outside the disk. We thus adopt a 1+1D approach. -1 ff 10 N2 e ects. In the case of photoprocesses for which cross section

On the one hand we solve the radiative transfer of FUV inter- ) -2 interstellar 1 10 H2O data is not available we have approximated the rate using a para- − OH s -3 1 stellar radiation as it propagates from outside the disk to the ( 10 Γ CO2 metric expression in which the rate , in units of s− , is expressed

e -4 1 au 100 au midplane along a series of vertical directions located at the dif- t 10 HCN as a function of the visual extinction A as a V r ferent radii of the grid described in Sect. 2.2. On the other hand 10-5 n

o -6 i 10 we solve the radiative transfer of FUV stellar radiation as it trav- t Γ = χα exp( γAV ), (7) a i 10-7 − els from the star through the disk along a series of directions c

o -8 s 10 11 given by a grid of 200 angles from the midplane (covering the s where χ is the FUV energy density with respect to that of the i -9 d 10 range from 0◦ to almost 90◦). The Meudon PDR code assumes o ISRF of Draine (1978), α is the rate under a given unattenu- t -10

o 10

h χ = ffi γ that stellar photons arrive from a direction perpendicular to the -11 ated radiation field with 1, and the coe cient controls the

p 10 plane-parallel surface of the cloud. In disks, the star may illumi- -12 Herbig Ae/Be disk decrease in the rate with increasing visual extinction. The coef- nate the disk with small grazing angles, in particular in the inner 10 ficients α and γ are specific of each photoprocess and depend regions where the flaring shape of the disk is less marked. It is 0.0 0.2 0.4 0.6 0.8 1.0 also on the spectral shape of the FUV field. Values of α and γ thus likely that when computing the FUV energy density along z/r corresponding to the ISRF have been taken from the databases the different directions from the star, at low penetration depths such as the UMIST database for astrochemistry (Woodall et al. Fig. 3. Contribution of the stellar (solid lines) and interstellar (dashed the Meudon PDR code underestimates the contribution of FUV Fig. 3. Contribution of the stellar (solid lines) and interstellar (dashed 2007; McElroy et al. 2013) and the OSU and KIDA databases lines)lines) FUV FUV fields fields to to the the photodissociation photodissociation rates rates of of various molecules photons scattered by dust from nearby regions around the disk asas a a function function of of the the height height over radius radius (z/r) at 1 au and 100 au in the (Wakelam et al. 2012, 2015), as well as from the compilation surface. However, it is not straightforward to properly correct by TT Tauri Tauri ( (upperupper panelpanel)) and Herbig AeAe/Be/Be (lower(lower panel)panel) disks. by van Dishoeck et al. (2006), recently revised by Heays et al. this geometrical effect without moving to 2D and therefore we (2017). do not apply any specific correction for it here. decrease in the rate with increasing visual extinction. The coef- In Fig. 3 we show the contribution of the stellar and inter- In summary, once the physical structure of the disk is cal- ficientsmake itα moreand γ versatileare specific and integrate of each photoprocessit into the protoplanetary and depend stellar radiation fields to the photodissociation rate of various culated at steady state (using the RADMC code as described in alsodisk on model. the spectral The Meudon shape ofPDR the code FUV solves field. Valuesthe FUV of radiativeα and γ molecules as a function of height over the disk midplane. This Sect. 2.1) and prior to the computation of the temporal evolution correspondingtransfer in one to dimension the ISRF for have a plane-parallel been taken from cloud the illuminated databases is shown at two radial distances from the star (1 au and 100 au) of the chemical composition, we use the Meudon PDR code to suchon one as side the UMIST by a FUVdatabase source. for In astrochemistry protoplanetary (Woodall disks, the et ge- al. for the T Tauri and Herbig Ae/Be disks. We see that in the upper- evaluate the photodissociation and photoionization rates at each 2007ometry; McElroy involves et two al. dimensions2013) and the (radial OSU and and vertical) KIDA databasesand there most regions of the disk, photoprocesses are clearly dominated location in the disk by calculating the FUV flux as a function (areWakelam two diff eterent al. 2012 FUV, 2015 sources,), as the well star asilluminating from the compilation from the by the stellar radiation field. However, as we go deeper into the of wavelength due to stellar and interstellar radiation and using bycentral van Dishoeck position and et al. the(2006 interstellar), recently radiation revised field by Heays illuminating et al. disk, at some point the contribution of the ISRF becomes more the relevant wavelength-dependent cross sections. Our approach (isotropically2017). from outside the disk. We thus adopt a 1+1D ap- important than the stellar one because of the more marked in- to treat photochemistry is thus different from other state-of-the- proach.In Fig. On3 thewe one show hand the we contribution solve the radiative of the stellar transfer and of inter- FUV crease of the visual extinction against stellar light than against art chemical models of protoplanetary disks in which the FUV stellarinterstellar radiation radiation fields as to it propagates the photodissociation from outside rate the ofdisk various to the interstellar photons. Therefore, depending on the location in the radiative transfer is solved in 2D but only in a few broad spectral moleculesmidplane along as a function a series of of height vertical over directions the disk located midplane. at the This dif- is disk, photodestruction can be driven by the ISRF or by the star. bands (e.g., Woitke et al. 2016) and it is in essence more similar shownferent radiiat two of radial the grid distances described from in the Sec. star 2.2. (1 Onau and the 100other au) hand for to the series of models by Walsh et al.(2012, 2014, 2015). the T Tauri and Herbig Ae/Be disks. We see that in the uppermost we solve the radiative transfer of FUV stellar radiation as it trav- 2.2.4. Reactions with vibrationally excited H We have compiled cross sections for 29 molecules and regionsels from of the the star disk, through photoprocesses the disk are along clearly a series dominated of directions by the 2 8 atoms (see AppendixA). The photodissociation rate of H 2 and stellargiven radiationby a grid field. of 200 However, angles from as we the go midplane deeper into (covering the disk, the at At the disk surface, the gas is strongly illuminated by FUV pho- CO are computed by solving the excitation and the line-by-line somerange point from the 0◦ to contribution almost 90◦). of The the MeudonISRF becomes PDR code more assumes impor- tons and vibrationally excited states of molecular hydrogen are radiative transfer taking into account self and mutual shielding tantthat than stellar the photons stellar one arrive because from a of direction the more perpendicular marked increase to the of easily populated through FUV fluorescence. Since the reactivity effects. In the case of photoprocesses for which cross section theplane-parallel visual extinction surface against of the cloud. stellar In light disks, than the against star may interstel- illumi- of H2 can be quite different when it is in the ground or in ex- data is not available we have approximated the rate using a larnate photons. the disk Therefore, with small dependinggrazing angles, on the in particular location in in the inner disk, cited vibrational states – the internal energy of H2 can be used 1 parametric expression in which the rate Γ, in units of s− , is photodestructionregions where the can flaring be driven shape by of the the ISRF diskor is lessby the marked. star. It is to overcome or diminish endothermicities or activation barriers expressed as a function of the visual extinction AV as thus likely that when computing the FUV energy density along which are present when H2 is in its ground vibrational state – we the different directions from the star, at low penetration depths have included a few reactions of H2 with specific rate constants Γ = χα exp( γA ), (7) 2.2.4. Reactions with vibrationally excited H2 − V the Meudon PDR code underestimates the contribution of FUV for each vibrational state. Specifically, we have included the re- Atphotons the disk scattered surface, by the dust gas from is strongly nearby illuminated regions around by FUV the pho- disk actions of H with C+ (important in the formation of CH+), He+, where χ is the FUV11 energy density with respect to that of the 2 tonssurface. and However, vibrationally it is excitednot straightforward states of molecular to properly hydrogen correct are by O, OH, and CN, with the rate constant expressions compiled by ISRF of Draine(1978), α is the rate under a given unattenuated easilythis geometrical populated ethroughffect without FUV fluorescence.moving to 2D Since and therefore the reactiv- we Agundez´ et al. (2010), and the reaction of H and S+, which may radiation field with χ = 1, and the coefficient γ controls the 2 itydonot of H apply2 can any be specific quite different correction when for it it here. is in the ground or 11 10 http://ism.obspm.fr in excitedIn summary, vibrational once states the physical – the internal structure energy of the of disk H2 can is cal- be The FUV is chosen to cover the wavelength range from the Lyman 11 The FUV is chosen to cover the wavelength range from the Lyman usedculated to overcome at steady or state diminish (using endothermicities the RADMC code or as activation described bar- in cutoff at 911.776 Å to the limit of the Habing field at 2400 Å. cutoff at 911.776 Å to the limit of the Habing field at 2400 Å. riers which are present when H2 is in its ground vibrational

A19, page 7 of 33 7 A&A 616, A19 (2018) state – we have included a few reactions of H2 with specific Table 2. Ice species, binding energies, and photodesorption yields. rate constants for each vibrational state. Specifically, we have included the reactions of H with C+ (important in the formation 2 Species E (K) Ref. Y (molecule photon 1) Ref. of CH+), He+, O, OH, and CN, with the rate constant expres- D − 3 sions compiled by Agúndez et al.(2010), and the reaction of H 2 CH4 1000 (1)♦ 10− (11) + + 3 and S , which may be important in the synthesis of the ion SH , C2H2 2587 (2)‡ 10− (11) 3 with the rate constant expressions calculated by Zanchet et al. C2H4 3487 (2)‡ 10− (11) (2013). The populations of the different vibrational states of H 3 2 C2H6 4387 (2)‡ 10− (11) are computed at each location in the disk with the Meudon PDR 3 H2O 5773 (3)♦ (1.3 + 0.032 Td) 10− (12) code. \ a b c × 3 ∗ O2 1161 (4)† (2.8 , 2.3 , 2.4 ) 10− (13) \ a b c × 3 § CO 1575 (5) † (9.5 , 4.8 , 10.3 ) 10− (14) 2.2.5. Adsorption processes \ a b c × 4 § CO2 2346 (4)† (8.2 , 2.0 , 0.98 ) 10− (15) × 3 §∗ The adsorption of gas species onto the surface of dust grains is H2CO 3260 (6)† 10− (11) a b c 4 treated in a rather simple and standard way. The adsorption rate CH3OH 4990 (7)♦ (1.5 , 1.1 , 1.3 ) 10− (16) 1 × 3 §∗ of a gas species i, in units of s− , is given by HCOOH 5570 (2)‡ 10− (11) 3 NH3 3830 (1)♦ 10− (11) ads \ a b c 3 Ri = αivi σdnd , (8) N2 1435 (5)† (2.0 , 1.6 , 0.89 ) 10− (13) h i × 3 § HCN 2050 (2)‡ 10− (11) where αi and vi are the sticking coefficient and thermal velocity ] 3 H2S 1945 (8) 10− (11) of each gas species i. The sticking coefficient is assumed to be 1 3 CS 1900 (2)‡ 10− (11) for all species and the thermal velocity is evaluated as √3kTk/mi, 3 H2CS 2700 (2)‡ 10− (11) where Tk is the gas kinetic temperature and mi is the mass of each 3 SO 2600 (2)‡ 10− (11) gas species i. The term σdnd is the product of the geometric 3 h i SO2 3900 (9)† 10− (11) cross section and the volume density of dust particles averaged 3 over the grain size distribution given by Eq. (2) and is evaluated OCS 3440 (10)♦ 10− (11) following the formalism of Le Bourlot et al.(1995). To simplify, \ Notes. ♦Pure ice; †amorphous water ice substrate ( submonolayer we only consider adsorption of a limited number of stable neutral ] species, among which there are the most typically abundant ice regime); solid SO2 substrate; ‡estimated for water ice substrate; constituents (see Table2). wavelength-dependent photodesorption yield is convolved over the 7–13.6§ eV range with the interstellar(a), T Tauri(b), and Herbig Ae/Be(c) FUV radiation fields described in Sect. 2.1.1; yields of direct photodes- 2.2.6. Desorption processes orption and fragmentation have been measured.∗ References. (1) Luna et al.(2014); (2) Garrod & Herbst(2006); Species which have been adsorbed on dust grains forming ice (3) Fraser et al.(2001); (4) Noble et al.(2012a); (5) Fayolle mantles can return to the gas phase through a variety of desorp- et al.(2016); (6) Noble et al.(2012b); (7) Doronin et al. tion mechanisms. In protoplanetary disks, the most important (2015); (8) Sandford & Allamandola(1993); (9) Schriver-Mazzuoli desorption processes are thermal desorption and photodesorp- et al.(2003), assuming that ED is directly proportional to the tion by FUV photons. We also include desorption induced by desorption temperature (see, e.g., Martín-Doménech et al. 2014); cosmic rays. Based on the results of experimental work using (10) Burke & Brown(2010); (11) assumed; (12) pure H 2O ice (>8 ML) isotopic markers (Bertin et al. 2012, 2013) and molecular dynam- in the 18–100 K range; the fraction of H2O molecules photodes- orbed is (0.42 + 0.002 Td), the remaining results in fragmentation into ics calculations (Andersson & van Dishoeck 2008), we consider 15 OH + H (Öberg et al. 2009); (13) pure O2 (30 ML) and N2 (60 ML) that only molecules from the top two monolayers can desorb effi- ices at 15 K (Fayolle et al. 2013); (14) pure CO ice (10 ML) at 18 K 13 ciently (Nl = 2). The term due to desorption can therefore be (Fayolle et al. 2011); (15) pure CO2 (10 ML) ice at 10 K; the fraction written in the kinetic rate equations as of CO2 molecules desorbed is 0.37, 0.42, and 0.22 under the interstel- lar, T Tauri, and Herbig Ae/Be FUV fields, respectively, the remaining dn dnice results in fragmentation into CO + O (Fillion et al. 2014); (16) pure i = i = (Rthd + Rpd + Rcrd)nice,desorbable, (9) dt − dt i i i i CH3OH (20 ML) ice at 10 K; the fraction of CH3OH molecules des- orbed is 0.08, while the fragmentation channels CH3 + OH, H2CO + H2, ice CO + H2 + H2 occur with fractions of 0.05, 0.07, and 0.80, almost inde- where ni and ni are the volume densities of species i in the gas thd pd crd pendently of the radiation field (Bertin et al. 2016; see also Cruz-Diaz and ice phases, respectively, Ri , Ri , Ri are the desorption et al. 2016). 1 rates, in units of s− , of thermal desorption, photodesorption, and desorption induced by cosmic rays, respectively (see below). The ice,desorbable – Thermal desorption. This process, which depends on the quantity ni is the volume density of species i in the top ice dust temperature Td and the binding energy of adsorption of each desorbable ice layers, which is simply equal to ni when these ice species ED, controls to a large extent the distribution of ices in layers are not fully occupied while it is given by ni multiplied protoplanetary disks and the location of the different snow lines ice 1 by the factor (ntop/ntot ) otherwise. The volume density of sites of each molecule. The thermal desorption rate, in units of s− , of in the top desorbable layers ntop is given by ns4 σdnd Nl (where an adsorbed species i is given by h 15i 2 ns is the surface density of sites, typically 1.5 10 cm− ; see ice ∼ × thd Hasegawa et al. 1992) and ntot is the sum of the volume densities Ri = ν0,i exp ( ED,i/Td), (10) ice − of all ice species, that is, i ni . This implementation of desorp- tion in the kinetic rate equations is similar to that of, for example, where ν0,i is the characteristic vibration frequency of the 2 Aikawa et al.(1999) and PWoitke et al.(2009); see also Cuppen adsorbed species i (evaluated as 2nskED,i/(π mi), where the et al.(2017). binding energy ED,i is expressed in units of K. Binding energies p A19, page 8 of 33 M. Agúndez et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars have been measured in the laboratory by depositing volatile efficiency of photodesorption is thus regulated by the ability species on different cold substrates and using temperature pro- of the molecules present in the ice surface and sub-surface grammed desorption methods (see Burke & Brown 2010 and to absorb FUV photons through electronic transitions. If these references therein). In general, binding energies show little transitions are dissociative, the situation becomes more com- dependence on the chemical or morphological nature of the plex because the fragments may desorb directly, recombine in substrate as long as the ice under study consists of various the ice, and then desorb or diffuse through the ice forming monolayers (ML). If desorption occurs at submonolayer cover- new molecules that may also desorb (e.g., Andersson & van age, desorption energies can be quite different depending on the Dishoeck 2008). Here we adopt a simple approach in which substrate; for example, they tend to be higher when employing ice molecules may desorb directly or as fragments upon FUV water ice as a substrate than when using silicates (Noble et al. irradiation, with yields based on experimental data for CO, N2, 2012a). The translation of these laboratory experiments into a O2,H2O, CO2, and CH3OH (see Table2). Desorption of frag- realistic model of thermal desorption in protoplanetary disks is ments has been observed upon irradiation of pure ices of H2O, complicated because ices are heterogeneous mixtures, thought CO2, and CH3OH (Öberg et al. 2009; Fillion et al. 2014; Martín- to be dominated by water ice but whose composition probably Doménech et al. 2015; Bertin et al. 2016; Cruz-Diaz et al. 2016), varies between cold and warmer regions. Also, molecules may and it is likely that desorption of dissociation fragments and new selectively co-desorb with other trapped species, experience vol- species formed in situ in the ice dominates over direct desorp- cano desorption following the crystallization of water ice, or tion for other polyatomic molecules. However, in the absence co-desorb with some of the major ice constituents (Collings et al. of experimental photodesorption yields for molecules other than 2004). Here we have adopted the simple and usual approach in CO, N2,O2,H2O, CO2, and CH3OH we have assumed that direct which the thermal desorption of each species is controlled by desorption dominates with assumed values for Yi. It is interest- a specific binding energy. We have collected values of experi- ing to note that photodesorption yields of CO, O2,N2, CO2, mentally measured ED, when possible using a water ice substrate and CH3OH have been measured as a function of wavelength and under a submonolayer regime. For those molecules for which using synchrotron techniques (Fayolle et al. 2011, 2013; Fillion experimental binding energies are not available, we have adopted et al. 2014; Bertin et al. 2016), which permits to compute the the values estimated by Garrod & Herbst(2006) for a water yield Yi under different FUV fields (see values for the ISRF and ice substrate based on the previous compilation by Hasegawa the T Tauri and Herbig Ae/Be stellar radiation fields in Table2). & Herbst(1993) and the experimental study of Collings et al. We note that the photodesorption yield of CO2, for example, is (2004). The binding energies of the ice molecules considered almost one order of magnitude higher under the ISRF than under and the corresponding references are given in Table2. a Herbig Ae/Be stellar field. – Photodesorption. The absorption of FUV photons of stellar – Cosmic-ray induced desorption. This mechanism is driven or interstellar origin (or generated through the Prasad Tarafdar by the impact of cosmic rays on dust grains. The energy mechanism) by icy dust grains can induce the desorption− of deposited on dust grains upon impact of relativistic heavy nuclei molecules on the ice surface. In regions where the dust tempera- of iron results in a local heating that induces the thermal desorp- ture is too cold to allow for thermal desorption, photodesorption tion of the ice molecules present in the heated region. According can provide an efficient means to bring ice molecules to the gas to Hasegawa & Herbst(1993), the desorption rate induced by 1 1 phase. The photodesorption rate, in units of s− , of an adsorbed cosmic rays, in units of s− , of a species i is given by species i is given by crd 19 thd R = 3.16 10− R (70 K), (12) i × i σ n Rpd = Y F h d di , (11) i i FUV 4 σ n n N where the numerical factor stands for the fraction of the time h d di s l spent by grains in the vicinity of a temperature of 70 K, at which where Yi is the yield of molecules desorbed per incident pho- much of the desorption is assumed to occur in the formalism 2 1 ton, FFUV is the FUV photon flux (in units of photon cm− s− ), of Hasegawa & Herbst(1993), and is derived adopting the Fe and the expression of Rpd, when inserted in Eq. (9), naturally flux estimated by Léger et al.(1985) for the local i interstellar medium and dust grains with a radius of 0.1 µm. The accounts for the fact that ices are not pure but consist of mul- thd tiple constituents and that desorption is only effective from the term Ri (70 K) is the rate of thermal desorption of species i, crd top desorbable layers (see, e.g., Cuppen et al. 2017). FUV pho- given by Eq. (10), evaluated at 70 K. The desorption rate Ri tons may have a stellar or interstellar origin (in which case scales with the cosmic-ray ionization rate. FFUV is evaluated with the Meudon PDR code at each position in the disk) or can be generated through the Prasad Tarafdar 2.2.7. Formation of H2 on grain surfaces −3 mechanism, in which case we adopt FFUV = 2 10 photon 2 1 4 × 2 1 The kinetics of H2 formation on grain surfaces in interstellar cm− s− (values between 750 and 10 photon cm− s− have space is usually described as been reported in the literature; e.g., Hartquist & Williams 1990; Shen et al. 2004). We note that this latter value scales with the dn(H ) 2 = R n n(H), (13) cosmic-ray ionization rate. Experiments carried out to study the dt f H photodesorption of pure ices or binary ice mixtures (Öberg et al. 2009; Muñoz Caro et al. 2010; Fayolle et al. 2011, 2013; Bertin where nH is the volume density of H nuclei, n(H) and n(H2) are et al. 2013; Fillion et al. 2014; Martín-Doménech et al. 2015) the volume densities of neutral H atoms and H2 molecules, and suggest that the main underlying mechanism, called desorption R f is the formation rate parameter for which usually the canoni- induced by electronic transition (DIET), involves absorption of cal value of 3 10 17 cm3 s 1 derived by Jura(1975) for diffuse × − − FUV photons in approximately the top five monolayers and elec- interstellar clouds is adopted. Here, we evaluate R f as tronic excitation of the absorbing molecules, followed by energy redistribution to neighboring molecules, which may break their 1 1 R f = S HH2 vH σdnd , (14) intermolecular bonds and be ejected into the gas phase. The 2 h inH

A19, page 9 of 33 A&A 616, A19 (2018) where vH is the thermal velocity of H atoms, evaluated as (Mandell et al. 2008; Pontoppidan et al. 2010a; Fedele et al. √3kTk/mH. The sticking coefficient of H atoms S H depends 2011; Salyk et al. 2011; Banzatti et al. 2017). Far-IR observa- on the gas kinetic temperature Tk and is evaluated through the tions with Herschel/PACS have essentially confirmed that the expression water detection rate is much higher in T Tauri disks than in disks around Herbig Ae/Be stars (Riviere-Marichalar et al. 2012; (1 + βTk/T0) Meeus et al. 2012; Fedele et al. 2012, 2013). Therefore, IR obser- S H(Tk) = S 0 β , (15) (1 + Tk/T0) vations suggest that water could be intrinsically less abundant in disks around Herbig Ae/Be stars than around T Tauri stars. Such where we have adopted S 0 = 1, T0 = 25 K, and β = 2.5, based on a trend is however not corroborated by our models. the experimental study carried out by Chaabouni et al.(2012) for As can be seen in Fig.4, in the T Tauri disk model, water is 4 a silicate surface. The recombination efficiency H2 in Eq. (14) present with fractional abundances of 10 in a surface layer at ∼ − depends on the dust temperature Td according to the expres- AV 1 in the inner 10 au from the star, while in the Herbig sion derived by Cazaux & Tielens(2002a,b) and is evaluated Ae/Be∼ disk, the warmer∼ temperatures cause the region of high with the parameters provided by Cazaux & Tielens(2002a) in H2O abundance to extend radially beyond 10 au and vertically their Table 1, with an updated value of 12 200 K for the des- down to the midplane. In the inner disk, water becomes very orption energy of chemisorbed H, as calculated by Goumans abundant in regions warmer than 200 K, where the reaction ∼ et al.(2009) for an olivine surface. Currently, the kinetics of OH + H2 is activated, and sufficiently shielded from FUV pho- grain-surface H2 formation considered in the model accounts tons, while OH is present in a thin layer on top of H2O resulting for the Langmuir Hinshelwood mechanism. In the future it from its photodissociation. In our generic T Tauri disk model, the will be worth considering− also the Eley Rideal mechanism, − inner midplane regions are not warm enough to sustain a high- which is expected to increase the H2 formation efficiency at water abundance, and thus most H2O (and all OH) are present in high gas temperatures (e.g., Le Bourlot et al. 2012; Bron et al. upper layers. We, however, note that the presence of water vapor 2014). in the inner midplane is very sensitive to the temperature and that other T Tauri disk models (e.g., Walsh et al. 2015) find high H2O 3. Results abundances in these regions. Therefore, the model predicts that as the stellar luminosity increases and the disk becomes warmer, In this section we present the calculated abundance distributions water vapor becomes more abundant. of various molecules in our fiducial T Tauri and Herbig Ae/Be The vertical column densities calculated for H2O and OH disk models, and compare them with available constraints from in the IR-observable atmosphere12 of the inner T Tauri disk are 17 18 2 15 2 observations, specifically highlighting the similarities and dif- 10 –10 cm− and 10 cm− , respectively, which are in the ∼ ferences between both types of disks. We focus on molecules low range of observed values (see left panel in Fig.5). The calcu- 2 3 that have been observed in disks at IR or (sub-)millimeter wave- lated OH/H2O column density ratio in the inner disk, 10− –10− , lengths. Detected species in disks have been summarized by is between the values derived in AA Tau, DR Tau, and AS 205A Dutrey et al.(2014). In Table3 we provide an updated com- (0.1–0.3; Carr & Najita 2008; Salyk et al. 2008) and that found 3 prehensive summary of the molecules observed in T Tauri and from a study of a much larger sample of T Tauri disks ( 10− ; ∼ Herbig Ae/Be disks. We first concentrate on molecules observed Salyk et al. 2011). Previous chemical models of inner T Tauri through IR observations, which are sensitive to the hot inner disks (Agúndez et al. 2008; Walsh et al. 2015) find H2O and OH disk: H2O and OH (Sect. 3.1), and simple organics such as C2H2, column densities and ratios of the same order of magnitude as the HCN, CH4, and CO2 (Sect. 3.2). We then focus on molecules ones calculated by us. In summary, chemical models of T Tauri observed at (sub-)millimeter wavelengths, which trace the outer disks predict the existence of a significant reservoir of hot water disk: the radicals C2H and CN (Sect. 3.3) and other organic in the inner regions (formed by warm gas-phase chemistry) and molecules with a certain complexity, such as H2CO (Sect. 3.4), smaller amounts of OH (formed by FUV photodissociation of the sulfur-bearing molecules CS and SO (Sect. 3.5), and molec- water), with numbers that are roughly in agreement with those ular ions (Sect. 3.6). We finally discuss the abundance distribu- derived from IR observations. tions of ices, for which most observational constraints consist of In disks around Herbig Ae/Be stars, OH has been detected determining the location of the CO snow line (Sect. 3.7). Abun- in about a dozen objects with a broad range of column densities dances are mostly expressed as column densities because this is (see Table3). In our Herbig Ae/Be disk model, the calculated 15 2 the quantity provided by most observational studies. Nonethe- vertical column density of OH is 10 cm− across the first 13 ∼ 2 less, sometimes we use the term fractional abundance, which 10 au, decreasing down to 10 cm− in the outer disk, in the ∼ hereafter refers to the abundance relative to the total number of low range of values derived from observations (see right panel in H nuclei. Fig.5). Walsh et al.(2015) calculate somewhat higher values for 16 17 2 the inner 10 au, 10 –10 cm− , more in line with the high range 3.1. Water and of observed values. Water has been convincingly detected at IR wavelengths only around one Herbig star, HD 163296 (Meeus In recent years, near-IR to sub-millimeter observations have et al. 2012; Fedele et al. 2012). The H2O and OH column densi- 14 15 2 provided important constraints on the presence of water and ties derived in this disk are similar, in the range 10 –10 cm− , its related radical (OH) in protoplanetary disks. At near- and and the emitting region for both species is constrained to be 15– mid-IR wavelengths, the spectra of disks around T Tauri stars 20 au from the star. In our Herbig Ae/Be disk model, the column show emission of hot H2O and OH arising from the inner disk density of water is very large in the inner disk (out to 4 au), (< a few au) atmosphere (Carr et al. 2004; Carr & Najita 2008, where it is very abundant in the midplane (see right panels∼ in 2011, 2014; Salyk et al. 2008, 2011; Pontoppidan et al. 2010a,b; Figs.4 and5), and experiences a sharp abundance decline with Mandell et al. 2012; Sargent et al. 2014; Banzatti et al. 2017), while in disks around Herbig Ae/Be stars, emission by OH is 12 We consider the IR-observable atmosphere to extend down to the relatively common but there is a striking lack of H2O emission AV = 10 layer, where the optical depth at 10 µm is of the order of unity.

A19, page 10 of 33 M. Agúndez et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars

Table 3. Summary of molecules (other than H2 and CO) observed in disks and abundances derived.

Species Spectral signature, region probed Detection rate, abundance, and references T Tauri Herbig Ae/Be (1,2,3,4,5,6,7,8,9,10,11) (4,7,11,12,13,14,15) H2O IR emission, inner atmosphere many disks one disk: HD 163296 (0.4 800) 1018 cm 2 (6,7) 1014 1015 cm 2 (14) − × − − − sub-mm emission, outer disk two disks: TW Hya, DG Tau (16,17,18) one disk: HD 100546 (18,19) 10 7 relative to H (16) ∼ − 2 OH IR emission, inner atmosphere many disks (2,3,4,6,7,8,11,15,20) a dozen of disks (4,7,11,12,21) 15 2 (6,7) 15 2 (12,15,21) (0.4 200) 10 cm− (0.01 200) 10 cm− − × (2,6,7,8,22,23,24) − × C2H2 IR emission/absorption, inner disk many disks none (0.5 70) 1015 cm 2 (6,7,22,23,24) − × − HCN IR emission/absorption, inner disk many disks (2,6,7,8,22,23,24) none (0.5 65) 1015 cm 2 (6,7,22,23,24) − × − mm emission, outer disk many disks (27,28,29,30,31,32,33,35) many disks (28,29,30,31,32,33,34) (0.2 10.6) 1012 cm 2 (28,31,32,33) (0.1 1) 1012 cm 2 (31,33,34) − × − − × − HNC mm emission, outer disk two disks: DM Tau, TW Hya (27,36) one disk: HD 163296 (36) a HNC/HCN = 0.3-0.4 (27) HNC/HCN = 0.1 0.2 (36) (25) − CH4 IR absorption, inner disk one disk: GV Tau none 17 2 (25) 2.8 10 cm− × (6,7,22,24,26) (7) CO2 IR emission/absorption, inner disk many disks one disk: HD 101412 16 2 (7,22,24,26) 16 2 (7) (0.04 10) 10 cm− 10 cm− − × (27,32,33,37,38) (33,34,37,39,40) C2H mm emission, outer disk many disks two disks: AB Aur, MWC 480 b (0.4 8.5) 1013 cm 2 (33,37) (0.6 1) 1013 cm 2 (33) − × − − × − CN mm emission, outer disk many disks (27,28,29,30,31,32,33) a handful of disks (28,29,30,31,33,34) 13 2 (28,31,32,33) 13 2 (28,31,33,34) (0.7 11.5) 10 cm− (0.1 1.5) 10 cm− − × (27,28,29,30,33,41,43,44) − × (30,33,34,42,45,46) H2CO mm emission, outer disk a dozen of disks a handful of disks 12 2 (33)c 12 2 (33)d (0.9 4.4) 10 cm− (1.6 3.3) 10 cm− − × 12 2 (47) − × CH3OH mm emission (ALMA), outer disk one disk: TW Hya; (3 6) 10 cm− none − × 12 2 (48) 12 2 (48,49) HC3N mm emission, outer disk two disks: GO Tau, LkCa 15; 10 cm− one disk: MWC 480; 10 cm− ∼ ∼ 13 2 (49) CH3CN mm emission (ALMA), outer disk none one disk: MWC 480; 10 cm− (38) ∼ 12 13 2 (50) c-C3H2 mm emission (ALMA), outer disk one disk: TW Hya one disk: HD 163296; 10 10 cm− (51) − NH3 sub-mm emission, outer disk one disk: TW Hya none (0.2 17) 10 11 relative to H (51) − × − 2 CS mm emission, outer disk a dozen of disks (27,32,33,52) two disks: AB Aur, MWC 480 (33,34,40) (1 20.9) 1012 cm 2 (32,33,52) (0.4 6.3) 1012 cm 2 (33,34) − × − − × − SO mm emission, outer disk two disks: CI Tau, GM Aur (33) one disk: AB Aur (33,34,40,42) (7.4 9) 1012 cm 2 (33) (0.5 10) 1012 cm 2 (33,34,42) − × − − × − HCO+ mm emission, outer disk many disks (28,29,30,33,35,53,54,55) many disks (28,29,30,33,34,54,56) 12 2 (28,33,53,54,55) 12 2 (28,33,34,54)e (0.3 20) 10 cm− (0.2 5) 10 cm− + − × (29,30,44,53,54,57) − × (45) N2H mm emission, outer disk a handful of disks one disk: HD 163296 f (0.1 30) 1012 cm 2 (53,54) 1.7 1011 cm 2 (45) − × − × − CH+ sub-mm emission, mid disk none two disks: HD 100546, HD 97048 (15,58) g 4.3 1012 cm 2 (58) × − (a) (b) Notes. Values are line intensity ratios rather than abundance ratios. Out of this range, Kastner et al.(2014) derive N(C2H) = 15 2 (c) 12 2 5.1 10 cm− in TW Hya. Out of this range, Aikawa et al.(2003) derive N(H2CO) = (7.2 19) 10 cm− in LkCa 15. (d) × 12 −(e) × Carney et al.(2017) derive a H 2CO abundance of (2 5) 10− relative to H2 in HD 163296. Out of this range, Mathews + 14 2 − × ( f ) + 14 et al.(2013) derive N(HCO ) = 1.5 10 cm− in HD 163296. Out of this range, Qi et al.(2013c) derive N(N2H ) = 10 15 2 (g) × + 16 17 2 − 10 cm− in TW Hya. In the same object, HD 100546, Fedele et al.(2013) derive N(CH ) = 10 –10 cm− for an emitting area inner to 50–70 au. References. (1) Carr et al.(2004); (2) Carr & Najita(2008); (3) Salyk et al.(2008); (4) Pontoppidan et al.(2010a); (5) Pontoppidan et al. (2010b); (6) Carr & Najita(2011); (7) Salyk et al.(2011); (8) Mandell et al.(2012); (9) Riviere-Marichalar et al.(2012); (10) Sargent et al.(2014); (11) Banzatti et al.(2017); (12) Fedele et al.(2011); (13) Meeus et al.(2012); (14) Fedele et al.(2012); (15) Fedele et al.(2013); (16) Hogerheijde et al.(2011); (17) Podio et al.(2013); (18) Du et al.(2017) ; (19) van Dishoeck et al.(2014); (20) Carr & Najita(2014); (21) Mandell et al.(2008); (22) Lahuis et al.(2006); (23) Gibb et al.(2007); (24) Bast et al.(2013); (25) Gibb & Horne(2013); (26) Kruger et al.(2011); (27) Dutrey et al.(1997); (28) Thi et al.(2004); (29) Öberg et al.(2010); (30) Öberg et al.(2011); (31) Chapillon et al.(2012a); (32) Kastner et al.(2014); (33) Guilloteau et al.(2016); (34) Fuente et al.(2010); (35) Fuente et al.(2012); (36) Graninger et al.(2015); (37) Henning et al.(2010); (38) Bergin et al.(2016); (39) Schreyer et al.(2008); (40) Pacheco-Vázquez et al.(2015); (41) Aikawa et al.(2003); (42) Pacheco-Vázquez et al.(2016); (43) Loomis et al.(2015); (44) Öberg et al.(2017); (45) Qi et al.(2013a); (46) Carney et al.(2017); (47) Walsh et al.(2016); (48) Chapillon et al.(2012b); (49) Öberg et al.(2015); (50) Qi et al.(2013b); (51) Salinas et al.(2016); (52) Dutrey et al.(2011); (53) Qi et al.(2003); (54) Dutrey et al.(2007); (55) Teague et al.(2015); (56) Mathews et al.(2013); (57) Qi et al.(2013c); (58) Thi et al.(2011). increasing radius. At 15–20 au from the star, where water is no derived in the HD 163296 disk. In the Herbig Ae disk model longer present in the midplane but in upper disk layers, the model of Walsh et al.(2015), at 10 au (the farthest radius studied by 15 16 2 14 15 2 yields N(H2O) = 10 –10 cm− and N(OH) = 10 –10 cm− , these authors) the column densities of H2O and OH are in the 16 17 with a OH/H2O ratio of 0.1, values which are not far from those range 10 –10 , with the OH/H O ratio approaching unity. The ∼ 2

A19, page 11 of 33 A&A 616, A19 (2018) Agundez´ et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars

1.0 1.0 -3 10 abundance relative to total H T Tauri Herbig Ae/Be 10-4 -5 0.8 H2O 0.8 H2O 10 10-6 10-7 0.6 0.6 10-8

r -9 / 10 z -10 0.4 0.4 10 10-11 10-12 0.2 0.2 10-13 10-14 0.0 0.0 10-15 100 101 102 100 101 102 1.0 1.0 -3 10 abundance relative to total H T Tauri Herbig Ae/Be 10-4 -5 0.8 OH 0.8 OH 10 10-6 10-7 0.6 0.6 10-8

r -9 / 10 z -10 0.4 0.4 10 10-11 10-12 0.2 0.2 10-13 10-14 0.0 0.0 10-15 100 101 102 100 101 102 r (au) r (au)

Fig.Fig. 4. 4. Calculated distributions of H2O and OH as as a a function function of of radius radiusrrandand height height over over radius radiusz/z/rrforfor the the T T Tauri Tauri (left) (left and panel Herbig) and Ae Herbig/Be (right) Ae/Be Agundez´ et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars (disks.right panel The dashed) disks. lines The dashedindicate lines the location indicate where the locationAV in the where outwardAV in vertical the outward direction vertical takes direction values of takes 0.01 values and 1. of 0.01 and 1.

3. Results21 2008,21 2011, 2014; Salyk et al. 2008, 2011; Pontoppidan et al. 10 T Tauri H2O 10 Herbig Ae/Be H2O )

2 20 2010a,b;20 Mandell et al. 2012; Sargent et al. 2014; Banzatti et al.

− OH OH In this10 section we present the calculated abundance distributions 2017),10 while in disks around Herbig Ae/Be stars, emission m 19 19 of variousc molecules in our fiducial T Tauri and Herbig AeC2/HBe2 C2H2

( 10 10

by OH is relatively common but there is a striking lack of

disky models, and compare them with available constraints from t 18 HCN 18 HCN i 10 H120O emission (Mandell et al. 2008; Pontoppidan et al. 2010a; observations,s specifically highlighting the similarities and dif- n 17 CH4 Fedele17 et al. 2011; Salyk et al. 2011; Banzatti et al. 2017).CH4 Far- e 10 10

ferencesd between both types of disks. We focus on molecules CO2 IR observations with Herschel/PACS have essentially confirmedCO2 thatn have1016 been observed in disks at IR or (sub-)millimeter wave- 1016

m HNC that the water detection rate is much higher in T Tauri disksHNC than

u 15 15 lengths.l 10 Detected species in disks have been summarized by 10 / o in disks around Herbig Ae Be stars (Riviere-Marichalar et al. c

Dutrey 1 et4 al. (2014). In Table 3 we provide an updated com- 14 l 10 2012;10 Meeus et al. 2012; Fedele et al. 2012, 2013). Therefore, a

prehensivec summary of the molecules observed in T Tauri and i 13 infrared13 observations suggest that water could be intrinsically t 10 10 Herbigr Ae/Be disks. We first concentrate on molecules observed e less abundant in disks around Herbig Ae/Be stars than around

v 12 12 through10 IR observations, which are sensitive to the hot inner T10 Tauri stars. Such a trend is however not corroborated by our disk:1 H0211O and OH (Sec. 3.1), and simple organics such as C2H2, models.1011 HCN, CH4, and100 CO2 (Sec. 3.2).10 We1 then focus on102 molecules 100 101 102 observed at (sub-)millimeter wavelengths,r (au) which trace the outer r (au) disk: the radicals C2H and CN (Sec. 3.3) and other organic = Fig.molecules 5. Calculated with verticala vertical certain column column complexity, densities densities such down down as to to H the the2COAAVV (Sec.=10 10 surface surface3.4), (solid (solid lines) lines) and and down down to to the the midplane midplane (dashed (dashed lines) lines) of of H H2O,2O, OH, OH, and and simple organics as a function of radius r for the T Tauri (left panel) and Herbig Ae/Be (right panel) disks. For HNC, only the total column density simplethe sulfur-bearing organics as a function molecules of radius CS andr for SO the (Sec. T Tauri 3.5), (left and panel molecu-) and HerbigAs Ae/Be can (right be seen panel in) disks. Fig. For 4, in HNC, the only T Tauri the total disk column model, density wa- down to the midplane is shown. Column densities densities derived derived from from observations observations are are indicated indicated by by vertical vertical lines lines (when (when there there are are ranges ranges4 of of values) values) or or lar ions (Sec. 3.6). We finally discuss the abundance distributions ter is present with fractional abundances of 10− in a surface byof squares ices, for (when which only most one observational value is available), constraints with with their their consist radial radial of locationsdeter- locations corresponding corresponding to to the the approximate approximate region region probed probed by∼ by observations observations (see (see Table3 3).). layer at AV 1 in the inner 10 au from the star, while in the mining the location of the CO snow line (Sec. 3.7). Abundances Herbig Ae/Be∼ disk, the warmer∼ temperatures cause the region of are mostly expressed as column densities because this is the high H2O abundance to extend radially beyond 10 au and ver- quantityThe vertical provided column by most densities observational calculated studies. for H Nonetheless,2O and OH whichtically are down not to far the from midplane. those derived In the in inner the disk,HD 163296 water becomes disk. In model of Walsh et al.(2015) and12 ours do a reasonable job at than to substantive differences in the chemistry between disks explaininginsometimes the IR-observable the we order use the of atmosphere magnitude term fractional ofof the the abundance, water inner and T Tauri OH which observa-disk here- are theveryaround Herbig abundant low- Ae and disk in intermediate-mass regions model warmer of Walsh than pre-mainet al.200 (2015), K, sequence where at 10 the stars. au reac- (the 17 18 2 15 2 ∼ 10after-10 referscm to− theand abundance10 cm− relative, respectively, to the total which number are in of the H farthesttion OH radius+ H2 studiedis activated, by these and authors) sufficiently the column shielded densities from FUV of tions of HD 163296.∼ It however remains puzzling to explain the Our model predicts that the16 reservoir17 of hot water present lownuclei. range of observed values (see left panel in Fig. 5). The cal- Hphotons,2O and OH while are OH in the is present range 10 in a-10 thin, layer with theontop OH/ ofH2 HO2O ratio re- extremely low detection rate of water in Herbig Ae/Be disks,2 as in the inner regions of disks around T Tauri and Herbig Ae/Be culated OH/H2O column density ratio in the inner disk, 10− - approachingsulting from unity. its photodissociation. The model of Walsh In our et generical. (2015) T Tauri and ours disk compared3 with T Tauri disks, taking into account the fact that stars vanishes typically beyond 10 au from the star, owing to 10− , is between the values derived in AA Tau, DR Tau, and AS domodel, a reasonable the inner job midplane at explaining regions the are order not warm of magnitude enough toof sus- the chemical3.1. Water models and hydroxyl (Walsh et radical al. 2015 and this work) predict that in thermal deactivation of the water-forming reaction OH + H2 disks205A around (0.1-0.3; Herbig Carr & stars, Najita water 2008; should Salyk be et even al. 2008)more abundant and that watertainand toa and high freeze-out OH water observations abundance,onto dust of grains. and HD thus163296. This most drastic It H however2O decline (and remains all of OH) vari- foundIn recent from years, a study near-IR of a much to sub-millimeter larger sample observations of T Tauri disks have puzzlingare present to explain in upper the layers. extremely We however low detection note that rate the of waterpresence in than in3 T Tauri disks. Several explanations have been proposed ous orders of magnitude in the abundance and column density (provided10− ; Salyk important et al. 2011). constraints Previous on the chemical presence models of water of inner and Herbigof water Ae vapor/Be disks, in the as inner compared midplane with is T very Tauri sensitive disks, taking to the tem-into (see∼ Antonellini et al. 2016 and references therein), most of of water (see Figs.4 and5) has been observationally probed whichTits Tauri related are disks related radical (Agundez´ to (OH) observational et in al. protoplanetary 2008; aspects Walsh et(e.g., disks. al. 2015) the At higher findnear- H level and2O accountperatureby mid- the and to fact far-IR that that other observationschemical T Tauri models disk in a (Walsh models few protoplanetary et (e.g., al. 2015 Walsh and etdisks this al. ofandmid-IR IR OH continuum column wavelengths, densities in Herbig the and spectra Ae/Be ratios of disks of disks the and same around the order lower T Tauri of sensitiv- magni- stars work)2015)(Blevins predictfind et high al. that 2016 H2O in). abundances disks The model,around in thesehowever, Herbig regions. stars, predicts Therefore,water that should there the itytudeshow reached as emission the ones for detection calculated of hot of H2 by emissionO us. and In OH summary, lines arising above chemical fromthe continuum) the models inner bemodelexists even an predicts more additional abundant that as reservoir the than stellar in of T Tauri cold luminosity waterdisks. inincreasesSeveral the outer explana- and parts the ofdisk T Tauri (< a disks few au) predict atmosphere the existence (Carr of et a al. significant 2004; Carr reservoir & Najita of tionsdisk becomeshave been warmer, proposed water (see vapor Antonellini becomes et al.more 2016 abundant. and refer- hot water in the inner regions (formed by warm gas-phase chem- ences therein), most of which are related to observational aspects A19,istry) page and 12 smaller of 33 amounts of OH (formed by FUV photodisso- (e.g., the higher level of infrared continuum in Herbig Ae/Be ciation of water), with numbers that are roughly in agreement disks and the lower sensitivity reached for detection of emis-11 with those derived from IR observations. sion lines above the continuum) than to substantive differences In disks around Herbig Ae/Be stars, OH has been detected in the chemistry between disks around low- and intermediate- in about a dozen objects with a broad range of column densi- mass pre-main sequence stars. ties (see Table 3). In our Herbig Ae/Be disk model, the calcu- 15 2 Our model predicts that the reservoir of hot water present lated vertical column density of OH is 10 cm− across the 13 ∼ 2 in the inner regions of disks around T Tauri and Herbig Ae/Be first 10 au, decreasing down to 10 cm− in the outer disk, in the low range of values derived∼ from observations (see right stars vanishes typically beyond 10 au from the star, owing to + panel in Fig. 5). Walsh et al. (2015) calculate somewhat higher thermal deactivation of the water-forming reaction OH H2 16 17 2 and to freeze-out onto dust grains. This drastic decline of var- values for the inner 10 au, 10 -10 cm− , more in line with the high range of observed values. Water has been convinc- ious orders of magnitude in the abundance and column den- ingly detected at IR wavelengths only around one Herbig star, sity of water (see Figs. 4 and 5) has been observationally probed by mid- to far-IR observations in a few protoplane- HD 163296 (Meeus et al. 2012; Fedele et al. 2012). The H2O and OH column densities derived in this disk are similar, in the tary disks (Blevins et al. 2016). The model however predicts range 1014-1015 cm 2, and the emitting region for both species that there exists an additional reservoir of cold water in the − / is constrained to be 15-20 au from the star. In our Herbig Ae/Be outer parts of T Tauri and Herbig Ae Be disks, typically be- disk model, the column density of water is very large in the in- yond 100 au and at intermediate heights (see Fig. 4). Water in these regions arises from the FUV photodesorption of wa- ner disk (out to 4 au), where it is very abundant in the mid- 7 ∼ ter ice and reaches peak fractional abundances of 10− , with plane (see right panels in Figs. 4 and 5), and experiences a sharp 13∼ 14 2 abundance decline with increasing radius. At 15-20 au from the typical vertical column densities of the order of 10 -10 cm− star, where water is no longer present in the midplane but in up- (see Fig. 5). This outer reservoir of water is also predicted by 15 16 2 previous chemical models of T Tauri disks that include pho- per disk layers, the model yields N(H2O) = 10 -10 cm− and N(OH) = 1014-1015 cm 2, with a OH/H O ratio of 0.1, values todesorption (e.g., Willacy & Langer 2000; Woitke et al. 2009; − 2 ∼ Semenov et al. 2011; Walsh et al. 2012), and has been detected 12 We consider the IR-observable atmosphere to extend down to the with Herschel/HIFI in the T Tauri disks TW Hya and DG Tau AV = 10 layer, where the optical depth at 10 µm is of the order of unity. (Hogerheijde et al. 2011; Podio et al. 2013) and in the Herbig Be

12 M. Agúndez et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars of T Tauri and Herbig Ae/Be disks, typically beyond 100 au is driven by ion-molecule routes. In these regions however the and at intermediate heights (see Fig.4). Water in these regions calculated abundance of CH4 could be especially uncertain if arises from the FUV photodesorption of water ice and reaches grain-surface chemistry (not included in our models) plays an 7 peak fractional abundances of 10− , with typical vertical col- important role. ∼13 14 2 umn densities of the order of 10 –10 cm− (see Fig.5). This The calculated vertical column densities of C2H2 and HCN outer reservoir of water is also predicted by previous chemi- in the IR-observable atmosphere of the inner T Tauri disk cal models of T Tauri disks that include photodesorption (e.g., (within 1 au from the star) are large, with maxima in the range 15 17 2 Willacy & Langer 2000; Woitke et al. 2009; Semenov et al. 2011; 10 –10 cm− , in good agreement with observed values (see Walsh et al. 2012), and has been detected with Herschel/HIFI left panel in Fig.5). Observations of T Tauri disks indicate that in the T Tauri disks TW Hya and DG Tau (Hogerheijde et al. C2H2 and HCN have similar abundances, although there is a sig- 2011; Podio et al. 2013) and in the Herbig Be disk HD 100546 nificant dispersion, and that they are somewhat less abundant 3 (van Dishoeck et al. 2014; Du et al. 2017), although it remains than water vapor (C2H2/HCN = 0.04–20; HCN/H2O = 10− – 1 elusive in many other protoplanetary disks (Bergin et al. 2010; 10− ; Lahuis et al. 2006; Gibb et al. 2007; Carr & Najita 2011; Du et al. 2017). It is remarkable that the detection rate of water Salyk et al. 2011; Mandell et al. 2012; Bast et al. 2013). These at sub-millimeter wavelengths is not so different between T Tauri observed ratios are in line with the values found in the T Tauri and Herbig Ae/Be disks as it is at IR wavelengths (see Table3), disk model. Methane has only been detected in one T Tauri disk, which suggests that the low detection rate of H2O IR emission GV Tau, in absorption (Gibb & Horne 2013). These authors in Herbig objects may not be due to an intrinsic deficit of water derive a column density of 2.8 1017 cm 2 and a rotational × − with respect to T Tauri systems. temperature of 750 K, which implies that the detected CH4 is distributed in the inner disk. In GV Tau, CH4 is somewhat more abundant than C2H2 and HCN. We note however that in DR Tau, 3.2. Simple organics: C2H2, HCN, CH4, and CO2 the non detection of CH4 in emission implies that it has an abun- It is known that there exists an important reservoir of simple dance similar to or smaller than C2H2 and HCN (Mandell et al. organic molecules in protoplanetary disks. Thanks to observa- 2012). In our T Tauri disk model, CH4 reaches a column den- tions at near- and mid-IR wavelengths, lines of molecules such sity of the order of those of C2H2 and HCN in the IR-observable as , , methane, and atmosphere of the inner disk (see left panel in Fig.5). Neither have been detected in absorption (Lahuis et al. 2006; Gibb et al. C2H2, HCN, or CH4 have been observed in Herbig Ae/Be disks, 2007; Gibb & Horne 2013; Bast et al. 2013) and in emission although our model predicts that they should be even more abun- (Carr & Najita 2008, 2011; Salyk et al. 2011; Kruger et al. dant than in T Tauri disks (see Figs.5 and6). Similar conclusions 2011; Mandell et al. 2012; Najita et al. 2013; Pascucci et al. are found in the models by Walsh et al.(2015). The lack of sim- 2013). These observations probe hot gas located in the inner (a ple organics in the spectra of Herbig Ae/Be disks has not been few au) disk, where these molecules are found in large abun- investigated to the extent of the lack of water, but it is likely dances. It is noteworthy that the vast majority of IR detections that observational rather than chemical effects are at the origin of simple organics correspond to disks around T Tauri stars of it. rather than to disks around Herbig Ae/Be stars, where neither Carbon dioxide has been extensively observed in T Tauri C2H2, HCN, nor CH4 are detected, and only CO2 has been disks but only in one Herbig Ae/Be disk, where the derived col- detected in one disk, HD 101412 (Salyk et al. 2011). It is there- umn density is within the range of values found in T Tauri disks. fore tempting to speculate that Herbig Ae/Be disks are less rich In our models, CO2 is formed abundantly, mostly in the inner in simple organics than disks around T Tauri stars. However, disk (<10 au in the T Tauri disk and <100 au in the Herbig disk) similarly to the case of H2O, we do not find such a trend in and over most of the vertical structure (see Fig.6). The forma- our models. tion of CO2 occurs in the gas-phase, mainly through the reaction In our T Tauri disk model, C2H2, HCN, and CH4 are formed OH + CO, and is less demanding in terms of temperature than 5 with high fractional abundances (a few 10− ) in the atmosphere the formation of C2H2, HCN, and CH4. Therefore, CO2 extends (A 1) of the inner (within a few au)× regions of the disk (see over larger radii than the other simple organics. The calculated V ∼ Fig.6). Their synthesis is driven by FUV photochemistry in a column density of CO2 in the IR-observable atmosphere of the 16 18 2 warm gas (see Agúndez et al. 2008; Bast et al. 2013; Walsh inner (<10 au) T Tauri disk is in the range 10 –10 cm− (see et al. 2015). In the Herbig Ae/Be disk model, the region over left panel in Fig.5), in the high range of observed values. As which these molecules have large fractional abundances extends with the other simple organics, according to the model there is to larger radii compared to the T Tauri disk, as a consequence no apparent chemical reason for a lower amount of CO2 in disks of the higher temperatures. Moreover, in the Herbig Ae/Be disk, around Herbig Ae/Be stars than in T Tauri disks. C2H2, HCN, and CH4 are formed abundantly in the inner mid- The simple organic molecules discussed here experience a plane, where the synthesis is not related to photochemistry but drastic decline in their column densities with increasing radius, to the fact that the chemical composition tends toward ther- especially for C2H2 and HCN (see Fig.5). This extended and mochemical equilibrium in these hot, dense, and FUV-shielded cooler reservoir of simple organics can be probed at millimeter regions. Both the T Tauri and Herbig Ae/Be disk models show wavelengths in the case of polar molecules like HCN. In fact, a progressive disappearance of simple organics as one moves HCN has been extensively characterized this way in protoplane- radially from the star, in this order: CH4,C2H2, and HCN. This tary disks (Dutrey et al. 1997; Thi et al. 2004; Fuente et al. 2010, behavior, already predicted by Agúndez et al.(2008), is a conse- 2012; Öberg et al. 2010, 2011; Chapillon et al. 2012a; Kastner quence of the requirements of temperature that each molecule et al. 2014; Guilloteau et al. 2016). Although the statistics of has to activate its corresponding formation routes, with CH4 Herbig Ae disks is low, these studies suggest that T Tauri disks being the most demanding. Such a trend is also found in the can retain somewhat larger HCN abundances in their outer parts disk models of Walsh et al.(2015) for C 2H2 and HCN. We note than Herbig Ae disks (see Table3). The column densities calcu- 13 2 that CH4, unlike C2H2 and HCN, is also predicted to be mod- lated for HCN beyond 100 au are 10 cm− in both the T Tauri erately abundant in cool midplane regions, where the synthesis and the Herbig Ae/Be disks, in the∼ high range of observed values

A19, page 13 of 33 A&A 616, A19 (2018) Agundez´ et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars

1.0 1.0 -3 10 abundance relative to total H T Tauri Herbig Ae/Be 10-4 -5 0.8 C2H2 0.8 C2H2 10 10-6 10-7 0.6 0.6 10-8

r -9 / 10 z -10 0.4 0.4 10 10-11 10-12 0.2 0.2 10-13 10-14 0.0 0.0 10-15 100 101 102 100 101 102 1.0 1.0 -3 10 abundance relative to total H T Tauri Herbig Ae/Be 10-4 -5 0.8 HCN 0.8 HCN 10 10-6 10-7 0.6 0.6 10-8

r -9 / 10 z -10 0.4 0.4 10 10-11 10-12 0.2 0.2 10-13 10-14 0.0 0.0 10-15 100 101 102 100 101 102 1.0 1.0 -3 10 abundance relative to total H T Tauri Herbig Ae/Be 10-4 -5 0.8 CH4 0.8 CH4 10 10-6 10-7 0.6 0.6 10-8

r -9 / 10 z -10 0.4 0.4 10 10-11 10-12 0.2 0.2 10-13 10-14 0.0 0.0 10-15 100 101 102 100 101 102 1.0 1.0 -3 10 abundance relative to total H T Tauri Herbig Ae/Be 10-4 -5 0.8 CO2 0.8 CO2 10 10-6 10-7 0.6 0.6 10-8

r -9 / 10 z -10 0.4 0.4 10 10-11 10-12 0.2 0.2 10-13 10-14 0.0 0.0 10-15 100 101 102 100 101 102 r (au) r (au)

Fig.Fig. 6. As in Fig.4 4 but but for for C C22H2,, HCN, HCN, CH CH4,, and and CO CO2..

(seedisk Fig. HD5 100546). The model (van Dishoeckpredicts a etslightly al. 2014; higher Du amount et al. 2017), of HCN al- tionsour model at near- suggest and mid-IR that HCN wavelengths, is somewhat lines more of moleculesabundant in such the inthough the outer it remains regions elusive of the T in Tauri many disk other compared protoplanetary to the Herbig disks asouter acetylene, regions ofhydrogen T Tauri cyanide, disks compared methane, to Herbigand carbon Ae/Be dioxide disks. disk.(Bergin It is et interestingal. 2010; Du to etnote al. that 2017). the It contrary is remarkable is found that in the haveWe, however, been detected caution in that absorption from an (Lahuis observational et al. 2006; perspective, Gibb et the al. innerdetection regions. rate The of water reason at is sub-millimeter that the chemical wavelengths synthesis is of not HCN so 2007;statistics Gibb of Herbig& Horne Ae 2013; disks Bastis low, et andal. 2013) from a and theoretical in emission one, isdiff differenterent between in nature T Tauri in the and hot Herbig inner disk Ae/Be than disks in the as cool it is outerat IR (Carrthe difference & Najita is 2008, small 2011; and could Salyk result et al. 2011; from the Kruger particular et al. 2011; set of regions.wavelengths In the (see outer Table disk 3), ( which>100 au), suggests HCN that is mainly the low present detection at Mandellparameters et al.adopted 2012; in Najita the models. et al. 2013; Pascucci et al. 2013). 8 intermediaterate of H2O IR heights emission with in fractional Herbig objects abundances may not of be10 due torela- an TheseHydrogen observations isocyanide, probe hotclosely gas locatedrelated to in HCN the inner from (a a few chemi- au) ∼ − tiveintrinsic to H2 deficit(see Fig. of water6). In with these respect regions, to T HCN Tauri is systems. formed by the disk,cal point where of view, these has molecules only been are detected found in around large abundances. the T Tauri stars It is same gas-phase chemical routes that operate in cold interstellar noteworthyDM Tau and that TW the Hya vast and majority the Herbig of infrared Ae star detections HD 163296 of ( simpleDutrey clouds, i.e., through ion-molecule reactions that lead to the pre- organicset al. 1997 correspond; Graninger to et disks al. 2015 around). The T Tauri lower stars detection rather than rateto of 3.2. Simple organics:+ C2H2, HCN, CH4, and CO2 cursor ion HCNH , which by dissociative recombination yields disksHNC aroundin disks Herbig compared Ae/Be to itsstars, most where stable neither isomer C2 HCNH2, HCN, suggests nor HCNIt is known as well that as its there isomer exists HNC. an important Thus, both reservoir observations of simple and CHthat4 HNCare detected, is less abundant and only thanCO2 HCN,has been although detected there in one is also disk, a organic molecules in protoplanetary disks. Thanks to observa- HD 101412 (Salyk et al. 2011). It is therefore tempting to spec- A19, page 14 of 33 13 M. Agúndez et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars Agundez´ et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars

1.0 1.0 -3 10 abundance relative to total H T Tauri Herbig Ae/Be 10-4 -5 0.8 C2H 0.8 C2H 10 10-6 10-7 0.6 0.6 10-8

r -9 / 10 z -10 0.4 0.4 10 10-11 10-12 0.2 0.2 10-13 10-14 0.0 0.0 10-15 100 101 102 100 101 102 1.0 1.0 -3 10 abundance relative to total H T Tauri Herbig Ae/Be 10-4 -5 0.8 CN 0.8 CN 10 10-6 10-7 0.6 0.6 10-8

r -9 / 10 z -10 0.4 0.4 10 10-11 10-12 0.2 0.2 10-13 10-14 0.0 0.0 10-15 100 101 102 100 101 102 r (au) r (au)

Fig.Fig. 7. AsAs in in Fig. Fig.4 4 but but for for C C22HH and and CN. CN.

selectionHNC in the effect hot as gas lines phase, of HNC in the have cool not outer been disk, targeted HNC as has often an dicateunits from that the C2H star. is presentConstraints with on an the abundance abundances similar of C2 toH that and asabundance those of HCN of the (e.g., order Öberg of that et of al. HCN 2011 because; Guilloteau the two et al. isomers 2016). ofCN CN mainly in most come disks, from with observations C2H/CN ratiosof a few in the protoplanetary range 0.2- Observationsshare the same indicate chemical that formation HNC is indeed routes, somewhat with the less molecular abun- 3.5disks, (Guilloteau the widely et studiedal. 2016), T Tauri and no disks significant DM Tau, di LkCafference 15, andbe- dantion HCNH than HCN,+ as main withHNC/HCN precursor. The ratios calculated in the range HNC 0.1–0.4/HCN ratio (see tweenTW Hya, T Tauri and the and Herbig Herbig Ae Ae disks disks. HD It 163296, is also MWC found 480, that and the Tablein the3 outer). These disk values (>100 may au) need is 0.3-0.4 to be in revised both the down T Tauri due andto dif- the CNAB Aur radical (Thi is et significantly al. 2004; Schreyer more abundant et al. 2008 than; HCNHenning in all et ob- al. ferencesHerbig Ae in/Be the disks, collisional in line excitation with the values rate coefficients derived from of obser- HCN served2010; Fuente disks, with et al. CN 2010/HCN; Chapillon abundance et al. ratios 2012a in; the Kastner range et4-30 al. andvations. HNC Interferometric (Sarrasin et al. observations 2010). The indicate calculated that distribution HNC does not of (Guilloteau2014) and from et al. the 2016), more and recent again study no of substantive Guilloteau di etfference al.(2016 be-), HNCextend approximately out to radii as follows large as that HCN of in HCN, TW Hya but (Graninger at a lower levelet al. tweenin which T Tauri a larger and sample Herbig Ae of disks disks. was In summary, observed. observations These studies at of2015), abundance a feature (see that Fig. is5 not). The predicted model by shows our T that Tauri there disk is model. a gap millimetersuggest that wavelengths C2H and CN tell can us reach that T higher Tauri disksabundances can retain in T larger Tauri inA possiblethe column explanation density of could HNC be whichrelated occurs to an enhanced at 1–10photodis- au in the abundancesdisks than in of Herbig C2H and Ae CNdisks in (see their Table outer3). regions This would than Herbig be in line Ae Tsociation Tauri disk cross (shifted section to of larger HNC radii compared in the to Herbig that of Ae/Be HCN, some- disk). disks,with the and general that in observational round numbers finding the C of2H a/CN higher ratio detection is 1 andrate Therething that is observational is suggested byevidence theoretical of such calculations a gap in (Chenel the distribu- et al. theof molecules CN/HCN in ratio T Tauri is disks10 in comparedboth types to of Herbig disk. It Ae is disks also∼ inter- (e.g., tion2016; of Aguado HNC from et al. SMA 2017). interferometric observations of TW Hya estingÖberg to et noteal. 2011 that; Guilloteau recent∼ ALMA et al. observations 2016). However, have found the confir- that (Graninger et al. 2015). While the presence of HNC in the hot Cmation2H emission of such shows hypothesis a ring-like is still distribution hampered by in the TWlow Hyastatistics and inner disk is related to the existence of large amounts of HCN, DMof Herbig Tau disks Ae disks (Bergin observed et al. 2016). to date. Observations indicate that a3.3. fraction Radicals of which C2H and isomerizes CN to HNC in the hot gas phase, C2H is present with an abundance similar to that of CN in most in the cool outer disk, HNC has an abundance of the order of disks,Inwith the T C TauriH/CN and ratios Herbig in the Ae range/Be disk 0.2 models,3.5 (Guilloteau C2H and et CN al. 2 − thatThe of radicals HCNC because2H and the CN, two usually isomers considered share the as same good chemi- tracers are2016 essentially), and no significant located in difference a relatively between thin layer T Tauri in the and disk Herbig sur- + calof regions formation affected routes, by with FUV the radiation molecular such ion as PDRs, HCNH areas among main face,Ae disks. between It is also the foundAV = that1 and theA CNV = radical0.01 islayers significantly (see Fig. more 7), 13 2 precursor.the most conspicuous The calculated molecules HNC/HCN detected ratio in protoplanetary in the outer disks disk withabundant vertical than column HCN in densities all observed of the disks, order with of 10 CN/HCNcm− in abun- the (at>100 millimeter au) is 0.3–0.4 wavelengths. in both These the T observations Tauri and the probe Herbig the Ae/Be emis- outerdance disk ratios (see in the Fig. range 8). Protoplanetary4–30 (Guilloteau disks et al. can 2016 be), seen and again as a disks,sion from in line the with outer the disk, values out derived to some from hundreds observations. of astronomi- Inter- PDRno substantive with the typical difference layered between structure T Tauri CO and/C/C Herbig+ along Ae the disks. ver- ferometriccal units from observations the star. indicate Constraints that HNCon the does abundances not extend of out C2 toH ticalIn summary, direction, observations and C2H and at millimeterCN become wavelengths abundant in tell the us layer that radiiand CN as large mainly as HCN come in from TW Hyaobservations (Graninger of eta few al. 2015 protoplane-), a fea- whereT Tauri neutral disks can atomic retain carbon larger reaches abundances its maximum of C2H abundance. and CN in turetary that disks, is not the predicted widely studied by our T T Tauri Tauri disks disk DMmodel. Tau, A LkCa possible 15, Thetheir formation outer regions of Cthan2H Herbig and CN Ae is disks, therefore and that associated in round to num- the explanationand TW Hya, could and be the related Herbig to Ae an disks enhanced HD 163296, photodissociation MWC 480, diskbers PDRthe C and2H/CN to the ratio availability is 1 and of the atomic CN/HCN C in ratioa FUV is illumi-10 in crossand AB section Aur (Thi of etHNC al. 2004; compared Schreyer to that et al. of 2008; HCN, Henning something et al. natedboth types gas. ofOur disk. results It is alsoregarding∼ interesting the T to Tauri note disk that recentare similar ALMA∼ to that2010; is suggestedFuente et al. by2010; theoretical Chapillon calculations et al. 2012a; (Chenel Kastner et al. 2016 et al.; thoseobservations of previous have modelsfound that of disksC2H emission around low-mass shows a ring-like young stars dis- Aguado2014) and et fromal. 2017 the). more recent study of Guilloteau et al. (2016), (e.g.,tribution Aikawa in the & TW Herbst Hya 1999b,and DM 2002; Tau disks Willacy (Bergin & Langer et al. 2016 2000;). in which a larger sample of disks was observed. These stud- vanIn Zadelho the Tff Tauriet al. and 2003; Herbig Willacy Ae/Be et al. disk 2006; models, Walsh C2 etH al. and 2010; CN ies suggest that C2H and CN can reach higher abundances in Semenov et al. 2011). There are some differences between the 3.3. Radicals C2H and CN are essentially located in a relatively thin layer in the disk sur- T Tauri disks than in Herbig Ae disks (see Table 3). This would Tface, Tauri between and the the HerbigAV = Ae 1/Be and disks.AV = The 0.01 higher layers gravity (see Fig. of7 the), 13 2 Thebe in radicals line with C2 theH and general CN, observational usually considered finding as of good a higher tracers de- Herbigwith vertical Ae/Be column star causes densities the layer of the containing order of C 102H andcm CN− in to the be oftection regions rate affected of molecules by FUV in T radiation Tauri disks such compared as PDRs, to are Herbig among Ae moreouter compressed disk (see Fig. toward8). Protoplanetary lower heights (see disks Fig. can 7), be although seen as the a thedisks most (e.g., conspicuousOberg¨ et al. molecules 2011; Guilloteau detected in et protoplanetary al. 2016). However, disks mostPDR significantwith the typical difference layered is that structure the calculated CO/C/C+ columnalong the densi- ver- atthe millimeter confirmation wavelengths. of such hypothesis These observations is still hampered probe by the the emis- low tiestical of direction, C2H and and CN C in2H the and outer CN disk become are somewhat abundant lower in the in layer the sionstatistics from of the Herbig outer Ae disk, disks out observedto some hundreds to date. Observations of astronomical in- Herbigwhere neutral Ae/Be disk atomic (see carbon Fig. 8). reaches The reason its maximum is that the Herbig abundance. disk A19, page 15 of 33 15 A&A 616, A19 (2018)

The formation of C2H and CN is therefore associated to the disk chemical models of T Tauri disks could still be reconciled if PDR and to the availability of atomic C in a FUV illuminated in the outer regions (>100 au) of the surface layer containing 5 6 3 gas. Our results regarding the T Tauri disk are similar to those C2H and CN (where densities are in the range 10 –10 cm− of previous models of disks around low-mass young stars (e.g., and gas kinetic temperatures are between 30 and 100 K) these Aikawa & Herbst 1999b; Aikawa et al. 2002; Willacy & Langer two radicals are subthermally excited. We note however that 2000; van Zadelhoff et al. 2003; Willacy et al. 2006; Walsh et al. this could have implications for the column densities derived 2010; Semenov et al. 2011). There are some differences between from observations under the assumption of local thermal equilib- the T Tauri and the Herbig Ae/Be disks. The higher gravity of rium (LTE). Dedicated non-LTE excitation and radiative transfer the Herbig Ae/Be star causes the layer containing C2H and CN to calculations (e.g., Aikawa et al. 2002) and comparison with be more compressed toward lower heights (see Fig.7), although millimeter observations are needed to shed light on this issue. the most significant difference is that the calculated column den- sities of C2H and CN in the outer disk are somewhat lower 3.4. Organic molecules of a certain complexity in the Herbig Ae/Be disk (see Fig.8). The reason is that the Herbig disk is illuminated by a more intense FUV field from In this section we briefly comment on various organic molecules the star than the T Tauri disk, and this narrows the photochemi- observed in disks that are more complex than those treated cally active layer and limits the ability of photochemistry to build in Sect. 3.2. We refer to H2CO and CH3OH, the hydrogena- molecules, resulting in smaller amounts of molecules specifi- tion descendants of , the cyanides HC3N cally formed by the action of photochemistry. This is a general and CH3CN, and the cyclic isomer of the hydrocarbon difference between disks around T Tauri and Herbig stars that C3H2. is better appreciated in the cool outer disk (>100 au), where has been quite commonly observed in proto- warm gas-phase chemistry is inhibited and thus cannot counter- planetary disks, with a higher detection rate in T Tauri disks than balance the effect of FUV photons. For example, beyond 100 au, in Herbig Ae disks (Öberg et al. 2010, 2011; Guilloteau et al. the radical OH (a species typically formed under the action of 2016), but very similar column densities, that is, a few times 12 2 photochemistry) is significantly more abundant in the T Tauri 10 cm− , in both types of disk (see Table3). Interferometric disk than in the Herbig disk (see Fig.5). In the case of C 2H observations of the disks DM Tau, TW Hya, and HD 163296 and CN, the fact that the abundances are lower in the Herbig have provided interesting constraints on the distribution and disk is linked to a slight defficiency in the abundance of neu- origin of H2CO (Qi et al. 2013a; Loomis et al. 2015; Öberg tral atomic carbon, which is a key starting point to form both et al. 2017; Carney et al. 2017). These observations point to C2H and CN. The calculated column densities of C2H and CN the presence of an inner component, consistent with gas-phase are in line with the values derived from observations. Moreover, formation, and an abundance enhancement in the outer disk the slight overabundance (a factor of a few) of these radicals in (beyond the CO snow line) resulting from formation on grain the T Tauri disk compared to the Herbig Ae/Be disk is in line surfaces by hydrogenation of CO ice followed by desorption. the observational suggestion that T Tauri disks can retain higher The recent detection of in the TW Hya disk (Walsh abundances of these two radicals. Another salient feature of the et al. 2016) also points to a similar outer disk origin driven by model is that the column densities of both C2H and CN increase the hydrogenation of CO ice. In our model, which does not with increasing radius in both the T Tauri and the Herbig Ae/Be include grain-surface chemistry, calculated column densities disks. The predicted inner gap could be consistent with the ring- in the outer regions of the T Tauri and Herbig Ae/Be disks are 12 2 like distribution found for C2H in the TW Hya and DM Tau disks around 10 cm− for H2CO (i.e., not far from observed values), (Bergin et al. 2016). These authors however propose a scenario but vanishingly small for CH3OH. We note that if most of the in which the ring morphology observed for C2H, and also for H2CO and CH3OH in the outer disk indeed comes from CO ice cyclic C3H2, is related to the evolution of ice-coated dust due through grain-surface chemistry, one should expect a significant to the combined effect of coagulation, gravitational settling, and differentiation between T Tauri and Herbig disks because the drift. latter are warmer and should have a lower reservoir of the The model indicates that HCN is much more abundant than precursor CO ice. CN in the inner regions of both types of disk, although as Other relatively large organic molecules observed in disks one moves away from the star, the CN/HCN ratio increases, are HC3N, CH3CN, and cyclic C3H2 (see Table3). Observa- reaching values above unity at radii >50 au (see Fig.8). The tions indicate that these molecules are present in the outer disk, overabundance of CN with respect to HCN in the outer disk out to a few hundred astronomical units. In these regions, our is in agreement with observations. According to the model, in model predicts column densities below the observed values by the outer disk, C2H and CN do not spatially coexist with HCN, one to two orders of magnitude. Calculated column densities for the radicals being exclusively present in a relatively thin layer at these molecules are quite different among chemical models in the disk surface while HCN is located at lower heights. There the literature. For example, for HC3N, Chapillon et al.(2012b) is some controversy regarding the region where C2H and CN calculate column densities one to two orders of magnitude above are present in T Tauri disks because millimeter observations the observed ones, while Walsh et al.(2014) find values ten derive low excitation temperatures (.10 K) for these two rad- times lower than observed. The dispersion of calculated col- icals (Henning et al. 2010; Chapillon et al. 2012a; Hily-Blant umn densities between different chemical models and the poor et al. 2017), which suggest that the emission could arise from agreement with observations suggests that the chemistry of these cold midplane regions rather than from the warm disk surface. If moderately complex molecules is not yet as robust as for smaller true, this would be in strong disagreement with the predictions of species.The probable role of grain-surface reactions in regulating our model and previous chemical models of T Tauri disks (e.g., their abundances makes it worth revisiting their chemistry in Aikawa & Herbst 1999b; Aikawa et al. 2002; Willacy & Langer T Tauri and Herbig Ae disks with an expanded chemical network 2000; van Zadelhoff et al. 2003; Willacy et al. 2006; Walsh including grain-surface chemistry. et al. 2010; Semenov et al. 2011), which also locate the radi- We note that our model predicts that organic molecules like cals C2H and CN well above the midplane. Observations and H2CO, CH3OH, HC3N, CH3CN, and c-C3H2 are enhanced in the

A19, page 16 of 33 M.AgAg Agúndezundez´undez´ et et al.:et al.: al.: The The The chemistry chemistry chemistry of of of disks disks disks around around around T T Tauri TTauri Tauri and and and Herbig Herbig Herbig Ae Ae Ae/Be/Be/Be stars stars stars

18 T Tauri C2H CS 18 Herbig Ae/Be C2H CS 10 18 T Tauri C2H CS 10 18 Herbig Ae/Be C2H CS ) 10 10 ) 2 2

− CN SO CN SO

− CN SO CN SO 17 17 m 17 17 m 10 10 c 10 HCN SO2 10 HCN SO2 c ( HCN SO2 HCN SO2

(

y y t 16 16 i t 16 16

i 10 10 s 10 10 s n n e

e 15 15 d

15 15 d 10 10

10 10 n n m

m 14 14 u 14 14 l 10 10 u

l 10 10 o o c

c l 13 13 l

a 10 13 10 13 a

c 10 10 i c i t t r

r 12 12 e 10 12 10 12 e

v 10 10 v

11 11 110011 110011 0 1 2 0 1 2 11000 11001 11002 11000 11001 11002 rr ( (aauu)) rr ( a(auu)) Calculated vertical column densities (down to the midplane) of the radicals C H and CN and some sulfur-bearing molecules as a function Fig.Fig. 8. 8. Calculated vertical column densities (down to the midplane) of the radicals C22H and CN and some sulfur-bearing molecules as as a a function function ofof radius radiusrrforfor the the T T Tauri Tauri (left ( (leftleftpanel) panel panel)) and and and Herbig Herbig Herbig Ae Ae/Be Ae//BeBe (right ((rightright panel) panelpanel)) disks. disks. HCN HCN is is also also shown shown to to allow allow for for the the visualization visualization of of the the CN CN CN/HCN/HCN/HCN ratio. ratio. ratio. TheThe ranges ranges of of column column densities densities derived derived from from observations observations are are are indicated indicated indicated by by the the the vertical vertical vertical lines lines plotted plotted in in the the outer outer disk disk (see (see Table Table 3).3 3).).

-3 1.0 1.0 -3 1.0 1.0 10 abundance relative to total H 10-4 abundance relative to total H TT TauriTauri HerbigHerbig Ae/Be Ae/Be 10 -4 10-5 CS CS 1100-5 0.80.8 CS 0.80.8 CS -6 10 -6 10-7 1100-7 0.6 0.6 -8 0.6 0.6 1100-8 r -9 r / 10 -9 / z 10

z -10 0.4 0.4 10 -10 0.4 0.4 10-11 10 -11 10-12 10 -12 0.2 0.2 10-13 0.2 0.2 10 -13 10-14 10 -14 10-15 0.0 0.0 10 -15 0.0 0 1 2 0.0 0 1 2 10 10 0 11001 10 2 10 0 11001 10 2 10 10 10 10 -3 1.0 1.0 -3 1.0 1.0 10 abundance relative to total H 10-4 abundance relative to total H T Tauri Herbig Ae/Be 10 -4 T Tauri Herbig Ae/Be 10-5 SO SO 10 -5 0.8 SO 0.8 SO 10-6 0.8 0.8 10 -6 10-7 10 -7 10-8 0.60.6 0.60.6 10 -8

r 10-9 r / 10 -9 / z 10

z -10 0.4 0.4 10 -10 0.4 0.4 10-11 10 -11 10-12 10 -12 0.2 0.2 10-13 0.2 0.2 10 -13 10-14 10 -14 10-15 0.0 0.0 10 -15 0.0 0 1 2 0.0 0 1 2 10 11000 11001 11002 11000 11001 11002 rr ((aauu)) rr ( (aauu)) Fig. 9. As in Fig. 4 but for CS and SO. Fig.Fig. 9. As in Fig.4 4 but but for for CS CS and and SO. SO. ofwarmof depletion depletion inner regions of of sulfur sulfur (within and and because abecause few au they fromthey can canthe star),be be used used in particular to to probe probe thesethesethen two hastwo molecules molecules been observed extend extend in down down various to to lower lower T Tauri heights. heights. disks The The and main main in for- for- two phenomenainphenomena the Herbig such such Ae/Be as as turbulence turbulence disk, following and and dust dust the traps traps abundance (Guilloteau (Guilloteau enhance- et et al. al. mationmationHerbig pathways Aepathways disks, to towhile both both SO molecules molecules was first involve involve observed various various in the fast fast Herbig neutral- neutral- Ae 2012;ment2012; of Pacheco-V Pacheco-V simple organicsazquez´azquez´ et etsuch al. al. 2016). as 2016). C2H CS2 CSand was was HCN among among (see the the Fig. first first5). neutralneutraldisk AB reactions reactions Aur and and wasand thermal laterthermal observed desorption desorption in a fromcouple from dust dust of T grains. grains. Tauri disksThe The moleculesSuch inner detected reservoir in of disks complex (Dutrey organics, et al. 1997) which and is since the result then calculated(see references column in Table densities3). These in the observations outer disk (beyond were carried 100 au) out molecules detected in disks (Dutrey et al. 1997) and since then calculated column12 densities2 in the outer disk (beyond 100 au) has been observed in various T Tauri disks and in two Herbig Ae are around 10 12cm− 2in both the T Tauri and the Herbig Ae/Be ofhas hot been gas-phase observed chemistry, in various Tcould Tauri be disks detectable and in two at millimeter Herbig Ae areat millimeter around 10 wavelengthscm− in both and the thus T Tauri trace and the the outer Herbig regions Ae/Be of disks,wavelengthsdisks, while while SO SO with was was ALMA first first observed observed provided in in the the Herbig angularHerbig Ae Ae resolution disk disk AB AB Aur Aurand disks,disks,disks. inAlthough in line line with with the the statisticsthe values values of derived derivedobserved from from disks observations observations is small, it seems (see (see and was later observed in a couple of T Tauri disks (see refer- Fig. 8). The S-bearing molecule SO2 has not been detected in sensitivityand was later are high observed enough. in a couple of T Tauri disks (see refer- Fig.that 8).both The CS S-bearing and SOmolecule have column SO2 densitieshas not been of the detected orderin of 12 13 2 encesences in in Table Table 3). 3). These These observations observations were were carried carried out out at at mil- mil- disksdisks10 –10 but but it itcm is is predicted− predictedand that to to have have there column column is no densities densities substantial of of the the difference same same or- or- limeterlimeter wavelengths wavelengths and and thus thus trace trace the the outer outer regions regions of of disks. disks. derderbetween as as or or slightly T slightly Tauri lowerand lower Herbig than than SO AeSO indisks in the the (see outer outer Table disk disk3 (see). (see Fig. Fig. 8). 8). In In Although3.5. Sulfur-bearing the statistics molecules: of observed CS disksand SO is small, it seems that the inner regions, the column densities of CS, SO, and SO2 are Although the statistics of observed disks is small, it seems12 that13 the innerIn our regions, model, the both column CS and densities SO are of mostly CS, SO, distributed and SO2 inare a both CS and SO have column densities of the order of 10 12-10 13 enhanced by various orders of magnitude, especially in the case Aboth couple2 CS and of SOsulfur-bearing have column molecules, densities of CS the and order SO, of have 10 -10 been enhancedlayer located by variousat intermediate orders of heights magnitude, (around especiallyAV 1), in both the in case the cm− 2 and that there is no substantial difference between T Tauri of the Herbig Ae/Be disk, although these regions∼ are likely to be detectedcm− and in that protoplanetary there is no substantial disks. The difference observation between of T theseTauri ofT Tauri the Herbig and the Ae/ HerbigBe disk, Ae/Be although disks these (see regions Fig.9). are In likely the inner to be andspeciesand Herbig Herbig is interesting Ae Ae disks disks (see (see because Table Table 3). 3).it provides information on the stronglystronglyregions spatially (within spatially a diluted diluted few au in in fromthe the millimeter millimeter the star) observations. observations. of the Herbig Ae/Be degreeInIn our our of depletion model, model, both both ofsulfur CS CS and and and SO SO because are are mostly mostly they distributed distributed can be used in in to a a disk,TheThe these column column two densities densitiesmolecules of of extend CS CS and and down SO SO calculated calculated to lower hereheights. here for for theThe the layerprobe located phenomena, at intermediate such as turbulence heights (around and dustA trapsV (1),Guilloteau both in Herbigmain formation Ae/Be disk pathways are similar to to both those molecules obtained involve in the chemi- various layer located at intermediate heights (around AV 1), both in Herbig Ae/Be disk are similar to those obtained in the chemi- theet al. T Tauri2012; and Pacheco-Vázquez the Herbig Ae/Be etdisks al. 2016 (see). Fig. CS was 9).∼∼ In among the inner the calfast model neutral of theneutral AB Aurreactions disk presented and thermal by desorption Fuente et al. from (2010) dust the T Tauri and the Herbig Ae/Be disks (see Fig. 9). In the inner cal model of− the AB Aur disk presented by Fuente et al. (2010) regionsfirstregions molecules (within (within adetected a few few au au from infrom disks the the star)( star)Dutrey of of the the et Herbigal. Herbig 1997 Ae Ae) and//BeBe disk,since disk, andandgrains. Pacheco-V Pacheco-V The calculatedazquez´azquez´ etcolumn et al. al. (2015). (2015). densities The The in main main the outer di difffferenceerence disk (beyond is is that that

A19, page 17 of 33 1717 A&A 616, A19 (2018)

12 2 + 100 au) are around 10 cm− in both the T Tauri and the 100 times smaller using IRAM PdBI. The chemistry of N2H + Herbig Ae/Be disks, in line with the values derived from obser- is relatively simple as it is formed by the reaction between H3 vations (see Fig.8). The S-bearing molecule SO 2 has not been and N2, which is favored in the coldest regions where CO is detected in disks but it is predicted to have column densities mostly in the form of ice, that is, in the outer midplane region. of the same order as or slightly lower than SO in the outer In our particular disk models, CO ice is present beyond a few disk (see Fig.8). In the inner regions, the column densities of tens of astronomical units in the T Tauri disk, while in the CS, SO, and SO2 are enhanced by various orders of magnitude, Herbig Ae/Be disk, due to the much warmer dust temperatures, especially in the case of the Herbig Ae/Be disk, although these it is barely formed just in the outer edge (see Sect. 3.7). As a + regions are likely to be strongly spatially diluted in the millimeter consequence, N2H is present in the outer T Tauri disk with a 11 2 observations. vertical column density of 10 cm− while it is almost absent ∼ The column densities of CS and SO calculated here for the in the Herbig Ae/Be disk (see Fig. 11). The vertical column den- + Herbig Ae/Be disk are similar to those obtained in the chemical sities calculated for N2H are in line with the low range of model of the AB Aur disk presented by Fuente et al.(2010) and observed values in the case of the T Tauri disk, while in the + Pacheco-Vázquez et al.(2015). The main difference is that those Herbig Ae/Be disk, N(N2H ) is lower than observed in models predict an enhancement in the column densities of CS HD 163296 by more than one order of magnitude (see Fig. 11). + and SO in the 100–200 au region, something that is not seen in The model also predicts that N2H must have a ring-like dis- the Herbig Ae/Be disk model presented here. The main reason of tribution with an inner gap, something that has been verified such difference is that those models assumed a more simplistic observationally in the TW Hya disk (Qi et al. 2013c). The inner + initial composition, with all the sulfur being initially in the form radius and vertical column density calculated for N2H are of CS, while in the present model we adopt a more realistic ini- highly dependent on which is the dust temperature structure tial composition in which sulfur is initially distributed in various across the disk, which in turn depends on parameters such as forms, mostly as S, S+, CS, and CS ice (see Sect. 2.2). The choice the stellar luminosity, the size and optical properties of dust of the initial composition can have non-negligible effects on the grains, and the disk geometry (flared vs. flat). In any case, since calculated abundances at ages typical of protoplanetary disks, disks around Herbig Ae/Be stars are expected to be significantly something that we plan to investigate in detail in the future. warmer than around T Tauri stars, one should expect larger + amounts of N2H in T Tauri disks than in Herbig Ae/Be disks. 3.6. Molecular ions + + Some deuterated ions, mostly DCO but also H2D and + The molecular ion HCO+ is probably one of the most widely N2D , have been observed at (sub-)millimeter wavelengths in observed species in protoplanetary disks (see references in a few protoplanetary disks (van Dishoeck et al. 2003; Ceccarelli Table3). In fact, it is remarkable that the detection rate of HCO + et al. 2004; Guilloteau et al. 2006; Qi et al. 2008; Öberg et al. is 100% in all the T Tauri and Herbig Ae disks targeted by Öberg 2010, 2011; Mathews et al. 2013; Huang & Öberg 2015; Teague et al.(2010, 2011) and by Guilloteau et al.(2016). Observed col- et al. 2015; Huang et al. 2017). Here we do not specifically 12 3 umn densities are of the order of 10 cm− , with no significant model deuterium chemistry (see, e.g., Willacy 2007; Teague difference between T Tauri and Herbig Ae disks (see Table3). In et al. 2015), but we point out that these molecular ions are usu- our model, HCO+ is mainly present in the outer disk at inter- ally present in the cool outer disk and are useful to probe the mediate heights (see Fig. 10), where it is mainly formed by fractional ionization. Estimates of the ionization fraction from + + the reaction between H and CO. The calculated vertical col- observations of H2D (which is present in the midplane of the 3 10 12 13 2 cold outer disk) are a few times 10− (Ceccarelli et al. 2004), umn density in the outer disk is in the range 10 –10 cm− in + both the T Tauri and the Herbig Ae/Be disks, in good agreement while values inferred from observations of DCO (which is expected in upper layers) are as high as 10 7 (Qi et al. 2008; with the values derived from observations (see Fig. 11) and in ∼ − line with results from previous chemical models of T Tauri disks Teague et al. 2015). In our T Tauri and Herbig Ae/Be disk mod- 10 (e.g., Aikawa & Herbst 1999b; Aikawa et al. 2002; van Zadelhoff els, the ionization fraction in the midplane ranges from 10− in the denser inner regions to 10 8 in the outer disk (see∼ Fig. 10). et al. 2003; Walsh et al. 2010, 2012; Semenov et al. 2011). Unlike ∼ − in the Herbig Ae/Be disk model, in the T Tauri disk model the In these midplane regions, the fractional ionization is controlled column density of HCO+ (and other molecular ions) experience by the cosmic-ray ionization rate and also depends on the gas a decline beyond 200 au. In these outer regions, the upper lay- density. As one moves to upper layers, where the gas is less ers are more exposed∼ to stellar FUV photons due to the flared dense, warmer, and less shielded against interstellar and stel- shape of the disk, and atomic ions are favored at the expense of lar FUV photons, the ionization fraction increases gradually up polyatomic ions, such as HCO+. This effect is probably a con- to very high values in the surface of the disk (see Fig. 10). sequence of the particular geometry of the disk model adopted According to the model, there are no substantive differences in here and may not be a general characteristic of T Tauri disks. the ionization degree of disks around T Tauri and Herbig Ae/Be Therefore, according to the model, there is no reason to expect stars. significantly different HCO+ column densities in T Tauri and The ion CH+ has been detected in a couple of disks, around Herbig Ae disks. the Herbig Be star HD 100546 and the Herbig Ae star HD 97048, Molecular ions other than HCO+ are difficult to observe in using Herschel (Thi et al. 2011; Fedele et al. 2013). The analy- protoplanetary disks. Nevertheless, sensitive observations with sis of the emission lines in HD 100546 indicates that CH+ has (sub-)millimeter interferometers have enabled the detection of an excitation temperature of 100–300 K, with most emission + + N2H in a few T Tauri disks and one Herbig Ae disk, HD 163296 arising from regions inside 100 au. In our model, CH is dis- (see references in Table3). Derived column densities are highly tributed along a surface layer on top of HCO+ (see Fig. 10), + dependent on each source and on each particular study. For where it is mainly formed by the reaction of C with hot H2 example, in the disk around LkCa 15, Qi et al.(2003) derive and FUV-pumped vibrationally excited H2 (Agúndez et al. 2010) + 13 2 7 N(N2H ) = 3.1 10 cm− from observations with the OVRO and reaches a maximum fractional abundance of 10− . The cal- + ∼ array, while Dutrey× et al.(2007) find a column density around culated distribution of CH is very similar in the T Tauri and

A19, page 18 of 33 M. Agúndez et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars Agundez´ et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars

1.0 1.0 -3 10 abundance relative to total H T Tauri Herbig Ae/Be 10-4 + + -5 0.8 HCO 0.8 HCO 10 10-6 10-7 0.6 0.6 10-8

r -9 / 10 z -10 0.4 0.4 10 10-11 10-12 0.2 0.2 10-13 10-14 0.0 0.0 10-15 100 101 102 100 101 102 1.0 1.0 -3 10 abundance relative to total H T Tauri Herbig Ae/Be 10-4 -5 0.8 e − 0.8 e − 10 10-6 10-7 0.6 0.6 10-8

r -9 / 10 z -10 0.4 0.4 10 10-11 10-12 0.2 0.2 10-13 10-14 0.0 0.0 10-15 100 101 102 100 101 102 1.0 1.0 -3 10 abundance relative to total H T Tauri Herbig Ae/Be 10-4 + + -5 0.8 CH 0.8 CH 10 10-6 10-7 0.6 0.6 10-8

r -9 / 10 z -10 0.4 0.4 10 10-11 10-12 0.2 0.2 10-13 10-14 0.0 0.0 10-15 100 101 102 100 101 102 r (au) r (au)

+ + Fig.Fig. 10. As in Fig.4 4 but but for for HCO HCO+Ag,, electrons, electrons,undez´ et al.: and and The CH CH+ chemistry.. of disks around T Tauri and Herbig Ae/Be stars

1015 1015 + + 2004), while valuesH inferredCO + from observations of DCOT Tauri(which calculatedHCO distribution+ of CH is very similarHerbig inAe/Be the T Tauri and ) + 7 + is expected2 in upperN2 layers)H are as high as 10− (Qi et al. 2008; HerbigN Ae2H/Be disks. The model predicts that the largest column − 1014 CH + ∼ 1014 CH + Teague et al.m 2015). In our T Tauri and Herbig Ae/Be disk mod- densities are reached in the outer disk (>100 au; see Fig. 11) c + + ( CH CH 3 10 3 els, the ionizationy fraction+ in the midplane ranges from 10− contrary to what+ observations of HD 100546 indicate. We how-

t HCNH HCNH i + s + 8 + 13 CF ∼ 13 CF in the densern 10 inner regions to 10− in the outer disk (see ever10 note that CH emission is probably very sensitive to the

e ∼ d

Fig. 10). In these midplane regions, the fractional ionization is temperature structure and geometry of each particular disk. In

n + controlled bym the12 cosmic-ray ionization rate and also depends on fact,12 CH has only been detected in a couple of disks from the u 10 10 l the gas density.o As one moves to upper layers, where the gas sample of 22 Herbig Ae/Be and 8 T Tauri systems surveyed by c

l

is less dense,a warmer, and less shielded against interstellar and Fedele et al. (2013). c

i 11 11 t 10 10 stellar FUVr photons, the ionization fraction increases gradually

e + up to veryv high values in the surface of the disk (see Fig. 10). Apart from HCO , the most abundant molecular ions in the + + + + According to the10 model, there are no substantive differences in disk10 are H , CH , CH (see Fig. 11), and C H , species which 10 10 3 2 3 2 2 0 1 2 0 1 + 2 the ionization degree10 of disks around10 T Tauri and Herbig10 Ae/Be are very10 difficult to detect.10 While H3 is1 mostly0 present in the stars. r (au) midplane of the outer disk,r ( theau) hydrocarbon cations are essen- tially present in the disk surface. For example, the formation of Fig. 11. 11.CalculatedCalculated+ vertical vertical column column densities densities (down (down to to the the midplane) midplane) of of selected selected+ molecular molecular+ ions ions as as a a function function of of radius radiusrrforfor the the T+ T Tauri Tauri (left (left panel) panel) and HerbigThe ion Ae CH/Be (righthas been panel) detected disks. Column in a couple densities of disks, derived around from observationsCH2 and are CH indicated3 is strongly by vertical linked lines to that or squares,of CH with(Agundez´ their radial et al. andthe Herbig Ae/Be Be star (right HD 100546 panel) disks. and the Column Herbig densities Ae star derived HD 97048, from observations2010). Molecular are indicated ions by not vertical yet observed lines or in squares, disks withbut potentially their radial locations corresponding corresponding to to the the approximate approximate region region probed probed by by observations observations (see (see Table Table3 3).). using Herschel (Thi et al. 2011; Fedele et al. 2013). The analy- detectable are HCNH+ (the precursor of HCN and HNC in the sis of the emission lines in HD 100546 indicates that CH+ has outer disk) and CF+ (formed in the disk PDR). Both ions are + 11 2 thosean excitation models predict temperature an enhancement of 100-300 in K,thewith column most densities emission of Npolar2H in and a few have T Tauri column disks densities and one in Herbig excess Ae of disk, 10 HDcm 163296− (see Herbig Ae/Be disks. The model predicts that the largest+ column has only been detected in a couple of disks from the sample of CSdensitiesarising and SO from are in reached regions the 100-200 in inside the au outer 100 region, disk au. Insomething(>100 our au; model, see that Fig. CH isnot 11is) seen con- dis- (seeFig.22 Herbig references 11), although Ae/Be in andTable detecting 8T 3). Tauri Derived them systems may column surveyed be challenging densities by Fedele are because highly et al. intributed the Herbig along Ae a/Be surface disk model layer presented on top of here. HCO The+ (see main Fig. reason 10), dependenttheir dipole on moments each source are not and very on high. each An particular additional study. interest- For trary to what observations of HD 100546 indicate.+ We, however, (2013). ofwhere such it di isfference mainly+ is formed that those by the models reaction assumed of C awith more hot simplis- H2 and example,ing feature in emerging the disk+ from around the LkCa model 15, is that Qi et most al. (2003) of the positive derive note that CH emission is probably very sensitive to the temper- Apart+ from HCO13 , the2 most abundant molecular ions in the ticFUV-pumped initial composition, vibrationally with all excited the sulfur H2 (Ag beingundez´ initially et al. in 2010) the+ Ncharge(N2H is) = not+3.1 carried+10 out+cm by− from molecular observations ions but with+ by atomic the OVRO ions ature structure and geometry of each particular disk. In fact,7 CH disk are H3 , CH×2 , CH3 (see Fig. 11), and C2H2 , species which formand reaches of CS, while a maximum in the present fractional model abundance we adopt aof more10 realistic− . The array,(metals while in the Dutrey disk midplane et al. (2007) and findcarbon a column in the disk density surface). around initial composition in which sulfur is initially distributed∼ in var- 100 times smaller using IRAM PdBI. The chemistry of N H+ is A19, page 192 of 33 ious forms, mostly as S, S+, CS, and CS ice (see Sec. 2.2). The relatively simple as it is formed by the reaction between H+ and 3 19 choice of the initial composition can have non-negligible effects N2, which is favored in the coldest regions where CO is mostly in on the calculated abundances at ages typical of protoplanetary the form of ice, that is, in the outer midplane region. In our par- disks, something that we plan to investigate in detail in the fu- ticular disk models, CO ice is present beyond a few tens of astro- ture. nomical units in the T Tauri disk, while in the Herbig Ae/Be disk, due to the much warmer dust temperatures, it is barely formed + just in the outer edge (see Sec. 3.7). As a consequence, N2H is 3.6. Molecular ions present in the outer T Tauri disk with a vertical column density 11 2 The molecular ion HCO+ is probably one of the most widely of 10 cm− while it is almost absent in the Herbig Ae/Be disk ∼ + observed species in protoplanetary disks (see references in (see Fig. 11). The vertical column densities calculated for N2H Table 3). In fact, it is remarkable that the detection rate of are in line with the low range of observed values in the case + HCO+ is 100 % in all the T Tauri and Herbig Ae disks targeted of the T Tauri disk, while in the Herbig Ae/Be disk, N(N2H ) by Oberg¨ et al. (2010, 2011) and by Guilloteau et al. (2016). is lower than observed in HD 163296 by more than one order of 12 3 + Observed column densities are of the order of 10 cm− , with no magnitude (see Fig. 11). The model also predicts that N2H must significant difference between T Tauri and Herbig Ae disks (see have a ring-like distribution with an inner gap, something that Table 3). In our model, HCO+ is mainly present in the outer disk has been verified observationally in the TW Hya disk (Qi et al. at intermediate heights (see Fig. 10), where it is mainly formed 2013c). The inner radius and vertical column density calculated + + for N2H are highly dependent on which is the dust temperature by the reaction between H3 and CO. The calculated vertical col- 12 13 2 structure across the disk, which in turn depends on parameters umn density in the outer disk is in the range 10 -10 cm− in both the T Tauri and the Herbig Ae/Be disks, in good agree- such as the stellar luminosity, the size and optical properties of ment with the values derived from observations (see Fig. 11) and dust grains, and the disk geometry (flared vs. flat). In any case, in line with results from previous chemical models of T Tauri since disks around Herbig Ae/Be stars are expected to be sig- nificantly warmer than around T Tauri stars, one should expect disks (e.g., Aikawa & Herbst 1999b, 2002; van Zadelhoff et al. + 2003; Walsh et al. 2010, 2012; Semenov et al. 2011). Unlike in larger amounts of N2H in T Tauri disks than in Herbig Ae/Be the Herbig Ae/Be disk model, in the T Tauri disk model the col- disks. + + + umn density of HCO (and other molecular ions) experience a Some deuterated ions, mostly DCO but also H2D + decline beyond 200 au. In these outer regions, the upper layers and N2D , have been observed at (sub-)millimeter wave- are more exposed∼ to stellar FUV photons due to the flared shape lengths in a few protoplanetary disks (van Dishoeck et al. 2003; of the disk, and atomic ions are favored at the expense of poly- Ceccarelli et al. 2004; Guilloteau et al. 2006; Qi et al. 2008; atomic ions such as HCO+. This effect is probably a consequence Oberg¨ et al. 2010, 2011, Mathews et al. 2013; Huang & Oberg¨ of the particular geometry of the disk model adopted here and 2015; Teague et al. 2015; Huang et al. 2017). Here we do not may not be a general characteristic of T Tauri disks. Therefore, specifically model deuterium chemistry (see, e.g., Willacy 2007; according to the model, there is no reason to expect significantly Teague et al. 2015), but we point out that these molecular ions different HCO+ column densities in T Tauri and Herbig Ae disks. are usually present in the cool outer disk and are useful to probe Molecular ions other than HCO+ are difficult to observe in the fractional ionization. Estimates of the ionization fraction + protoplanetary disks. Nevertheless, sensitive observations with from observations of H2D (which is present in the midplane 10 (sub-)millimeter interferometers have enabled the detection of of the cold outer disk) are a few times 10− (Ceccarelli et al.

18 A&A 616, A19 (2018)

+ are very difficult to detect. While H3 is mostly present in the water ice extends down to the inner disk edge is a particular out- midplane of the outer disk, the hydrocarbon cations are essen- come of the T Tauri disk model adopted here, which results in tially present in the disk surface. For example, the formation dust that is too cold to allow for efficient thermal desorption of + + + of CH2 and CH3 is strongly linked to that of CH (Agúndez water ice in the inner disk midplane. Since dust temperatures are et al. 2010). Molecular ions not yet observed in disks but poten- strongly dependent on parameters such as the size of dust grains, tially detectable are HCNH+ (the precursor of HCN and HNC other T Tauri disk models may find water snow lines that are fur- in the outer disk) and CF+ (formed in the disk PDR). Both ions ther out compared to that obtained here (see, e.g., Walsh et al. 11 2 are polar and have column densities in excess of 10 cm− (see 2015). An obvious difference between the T Tauri and the Her- Fig. 11), although detecting them may be challenging because big Ae/Be disk models is that in the latter, snow lines shift to their dipole moments are not very high. An additional interest- larger radii owing to the warmer dust temperatures (see Fig. 12), ing feature emerging from the model is that most of the positive and as a consequence, the mass of ices in the disk becomes charge is not carried out by molecular ions but by atomic ions smaller. (metals in the disk midplane and carbon in the disk surface). The abundance distributions of CO and H2O ices shown in Fig. 12 serve to illustrate two extreme cases of molecules with 3.7. Ices and snow lines low and high binding energies, respectively (see Table2). Other ices with binding energies between those of CO and H2O have Ices account for an important percentage of the matter of pro- intermediate distributions between those of these two molecules. toplanetary disks. The high densities and cold temperatures In general, the smaller the binding energy, the larger the radius prevailing in the midplane regions ensure a rapid and efficient to which the snow line shifts. This relationship is illustrated in adsorption of gas-phase molecules onto dust grains, where they Fig. 13, where we plot the radius at which the midplane snow line settle and live long in the form of ice. The presence or absence lies for various ices as a function of their binding energies. There of a particular type of ice in a certain disk region depends on is a clear trend which indicates that both quantities are related by q the balance between adsorption and desorption, and, because a power law of the type ED rms− , where rms stands for radius adsorption rates are similar for most molecules (see Sect. 2.2.5), of midplane snow line. A fit∝ to the data points in Fig. 13 yields the extent of each particular type of ice is determined by its an exponent q of 0.23 in both the T Tauri and the Herbig Ae/Be specific desorption rate. According to the model, among the vari- disks. This behavior∼ is not surprising since the midplane snow ous desorption mechanisms considered (see Sect. 2.2.6), thermal line of a given ice is essentially located at the radius at which desorption is clearly the most important and the one that shapes the dust temperature in the midplane becomes similar to the the bulk distribution of most ices. We however note that for those condensation temperature of the ice, and the latter is directly pro- species with large binding energies, the vertical extent of ices portional to the binding energy of the ice. That is, the observed in the outer disk is essentially controlled by photodesorption by relation between binding energy and midplane snow line merely interstellar and stellar FUV radiation. In these outer regions, dust reflects how the midplane dust temperature varies with radius. In temperatures may not be high enough to trigger thermal desorp- Fig. 13 we have overplotted as a solid line the radial profile of the tion of strongly bonded ices, while FUV photons can penetrate dust temperature in the midplane scaled up by a factor of 45. The down to intermediate heights and provide an efficient means of comparison between solid line and data points in Fig. 13 sug- desorbing ice molecules. The main effect of photodesorption is gests that a factor of proportionality between binding energy and that it shifts down the snow line13 of highly polar molecules in condensation temperature in the range 30–50 is adequate for the the outer disk, enhancing their gas-phase abundance at inter- protoplanetary disks modeled here. For comparison, Hollenbach mediate heights. In summary, the distribution of ices with low et al.(2009) calculate this factor of proportionality to be 50 ∼ binding energies, such as CO and N2, is controlled by thermal while Martín-Doménech et al.(2014) quote a value of 30 (from ∼ desorption, while for ices with large binding energies, like NH3 Attard & Barnes 1998). and H2O (see Table2), distribution is determined by thermal Using observations to reveal the location of snow lines in desorption in the inner regions and by photodesorption in the protoplanetary disks is still challenging, although ALMA obser- outer disk. Other desorption mechanisms, such as cosmic-ray vations are starting to permit interesting constraints. For exam- induced desorption or photodesorption by FUV photons gen- ple, in the disk around the T Tauri star TW Hya the CO snow line erated through the Prasad Tarafdar mechanism, are much less has been found to lie at a radius of 20 au (Schwarz et al. 2016; − important. Zhang et al. 2017), while in the disk∼ around the Herbig Ae star In protoplanetary disks, ices are mostly distributed around HD 163296 it has been located at a radius of 90 au from the star the midplane, from the very outer disk down to an inner edge, (Qi et al. 2015). These two observational points are included in which is given by the location of the snow line in the midplane. Fig. 13. While the CO snow line derived for TW Hya is close to Since the midplane snow line is essentially controlled by thermal the value calculated in our T Tauri disk model, the radius derived desorption (and thus by the binding energy of the particular ice for HD 163296 is much smaller than indicated by our Herbig and by the dust temperature structure of the disk along the mid- Ae/Be disk model, which puts the CO snow line even beyond plane), ices appear progressively in the radial direction according the outer disk edge. We however note that the coincidence in the to their binding energies. In Fig. 12, we show the calculated dis- case of TW Hya is very likely accidental because our generic tributions of CO and H2O ices in the T Tauri and Herbig Ae/Be T Tauri disk model is not aimed to represent either the TW Hya disks. First focusing on the T Tauri disk model, we see that CO disk or any other particular disk. The observational finding of ice, for which the adopted binding energy is 1575 K, is only a CO snow line farther out in the HD 163296 disk than in the present beyond a few tens of astronomical units, while H2O ice, TW Hya disk is in line with the general expectation that snow which has a much higher binding energy (the adopted value is lines are shifted to larger radii in Herbig Ae/Be disks compared 5773 K), is present all over the disk midplane. The fact that to T Tauri disks, although taking into account the poor statistics, consisting of just one object of each type, this result may be 13 The snow line of a particular species is defined here as the transition fortuitous. Expanding the sample of disks seems mandatory to region where its gas and ice abundances become equal. draw more definitive conclusions. It will also be very interesting

A19, page 20 of 33 M. Agúndez et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars Agundez´undez´ et et al.: al.: The The chemistry chemistry of of disks disks around around T T Tauri Tauri and and Herbig Herbig Ae Ae/Be/Be stars stars

1.0 1.0 -3-3 1.0 1.0 abundance relative to total H 1100 abundance relative to total H T Tauri HerbigHerbig Ae/Be Ae/Be 1100-4-4 -5-5 0.8 CO ice 0.80.8 COCO ice ice 1100 1100-6-6 1100-7-7 0.6 0.60.6 1100-8-8 r r -9-9 / / 1100 z z -1-010 0.4 0.40.4 1100 1100-1-111 1100-1-212 0.2 0.20.2 -13 1100-13 1100-1-414 0.0 0.00.0 1100-1-515 100 110011 110022 110000 110011 110022 1.0 1.0 -3-3 1.0 1.0 10 abundance relative to total H 10 abundance relative to total H T Tauri HerbigHerbig Ae/Be Ae/Be 1100-4-4 -5-5 H22O iice HH22OO i cicee 1100 0.8 0.80.8 -6 1100-6 -7 1100-7 -8 0.6 0.60.6 1100-8 r r -9-9 /

/ 10

z 10 z -10 0.4 0.4 1100-10 0.4 0.4 -11 1100-11 -12 1100-12 0.2 0.20.2 -13 1100-13 -14 1100-14 0.0 0.0 -1-515 0.0 0 1 2 0.0 0 1 2 1100 10 0 11001 11002 11000 11001 11002 rr ((aauu)) rr ( a(auu))

Fig. 12. Calculated distributions of CO and H2O ices as a function of radius r and height over radius z/r for the T Tauri (left) and Herbig Ae/Be Fig.Fig. 12. CalculatedCalculated distributions distributions of of CO CO and and H H2O2O ices ices as as a function a function of radiusof radiusr andr and height height over over radius radiusz/r forz/r thefor T the Tauri T Tauri (left panel (left)) and and Herbig Herbig Ae Ae/Be/Be ((right)right panel disks.) disks. The dashed The dashed line line in in line each each in panel panel each indicates panelindicates indicates the the location location the location of of the the corresponding of corresponding the corresponding snow snow line. line. snow line.

7000 desorptiondesorptionabundance rate. rate.distributions According According can to to be the the sensitive model, model, among to among many the the of various them. various In des- des- this H2O 6000 H2O orptionorptionstudy we mechanisms mechanisms put the focus considered considered on the photochemistry (see (see Sec. Sec. 2.2.6), 2.2.6), and thermal thermal thus desorp- heredesorp- we 5000 tiontionevaluate is is clearly clearly the influencethe the most most important important of a couple and and of the the related one one that that aspects: shapes shapes the the bulk spec- bulk distribution of most ices. We however note that for those species 4000 distributiontrum of the of star most and ices. the We way however in which note photodestruction that for those species rates 4000 HerbigHerbig Ae/BeAe/Be NH3 NH3 with large binding energies, the vertical extent of ices in the outer NH3 NH3 withare computed. large binding We energies,concentrate the on vertical the T extentTauri disk of ices model in the for outer this 3000 H2CO T Tauri H2CO disk is essentially controlled by photodesorption by interstellar 3000 H2CO T Tauri H2CO disksensitivity is essentially analysis. controlled by photodesorption by interstellar andandT stellar stellar Tauri FUV FUV stars radiation. radiation. are cool, In In with these these effective outer outer regions, regions, temperatures dust dust temper- temper- of the HCN HCN CO2 HCN CO2 HCN atures may not be high enough to trigger thermal desorption of 2000 CO2 CO2 aturesorder of may 4000 not K, be although high enough they tousually trigger have thermal an important desorption FUV of 2000 HD 163296 strongly bonded ices, while FUV photons can penetrate down HD 163296 stronglyexcess that bonded can greatly ices, while affect FUV the photons photochemistry can penetrate of the down disk. binding energy (K) TW Hya binding energy (K) ffi TW Hya CO to intermediate heights and provide an e fficient means of des- CO toTo intermediate evaluate the heights influence and of provide this FUV an excess e cient we means compare of des- our N2 orbing ice molecules. The main effect of photodesorption is that N2 O2 ff O2 orbingfiducial ice T Taurimolecules. disk Themodel, main13 in e whichect of we photodesorption consider the isstellar that CH4 it shifts down the snow line 13of highly polar molecules in the 1000 CH4 itspectrum shifts down of TW the Hya, snow with line a modelof highly in whichpolar molecules we assume in that the 1000 outerouterthe star disk, disk, emits enhancing enhancing as a blackbody their their gas-phase gas-phase at a temperature abundance abundance of at at intermediate4000 intermediate K (both 800 800 100 101 102 heights. In summary, the distribution of ices with low binding 100 101 102 heights.spectra are In shownsummary, in Fig. the1 distribution). The lack of of FUV ices excesswith low in this binding lat- radius of midplane snowline (au) energies, such as CO and N2, is controlled by thermal desorp- radius of midplane snowline (au) energies,ter case results such as in CO much and lower N2, is photodestruction controlled by thermal rates (compare desorp- tion,tion,our values while while infor for Table ices ices withB.1 with with large large those binding binding of van energies, energies, Dishoeck like like et NH NH al.3 2006andand, Midplane snow line of various ices (along x-axis) as a func- 3 Fig.Fig. 13. Midplane snow line of various ices ices (along (along xx-axis)-axis) as as a a func- func- H2O (see Table 2), distribution is determined by thermal desorp- tionFig. of13. the binding energy (along y-axis) in the T Tauri (green circles) Hor2O see (see Heays Table et 2), al. distribution 2017). The is net determined effect is by that thermal without desorp- FUV tion of the binding energy (along (along yy-axis)-axis) in in the the T T Tauri Tauri (green (green circles) circles) tion in the inner regions and by photodesorption in the outer disk. and Herbig Ae/Be/ (magenta circles) disks. Observational CO snow lines tionexcess, in the most inner photodestruction regions and by photodesorptionrates are dominated in the by outer the ISRF disk. and Herbig Ae/Be (magenta circles) disks. disks. Observational Observational CO CO snow snow lines lines Other desorption mechanisms, such as cosmic-ray induced des- in TW Hya (Schwarz et al. 2016; Zhang et al. 2017) and HD 163296 Otherratherdesorption than by stellar mechanisms, radiation. such As as as example, cosmic-ray in the induced left panel des- in TW Hya (Schwarz et et al. al. 2016; 2016; Zhang Zhang et et al. al. 2017) 2017) and and HD HD 163296 163296 orption or photodesorption by FUV photons generated through (Qi et al. 2015) are also shown. We also plot the dust temperature in orptionof Fig. 14 or we photodesorption show the photodissociation by FUV photons rate generated of CO as through a func- (Qi et al. 2015) are also shown. shown. We We also also plot plot the the dust dust temperature temperature in in the Prasad-Tarafdar mechanism, are much less important. the midplane (scaled up by a factor of 45) as a function of radius in the thetion Prasad-Tarafdar of height at a radius mechanism, of 1 au, whereare much both less the important. contributions of the midplane (scaled up up by by a a factor factor of of 45) 45) as as a a function function of of radius radius in in the the TT Tauri Tauri (green solid line) and Herbig AeAe/Be/Be (magenta solid line) line) disk disk In protoplanetary disks, ices are mostly distributed around T Tauri (green solid line) and Herbig Ae/Be (magenta solid line) disk the ISRFIn protoplanetary and the star are disks, taken ices into are account. mostly It distributed is seen that around in the models.models. the midplane, from the very outer disk down to an inner edge, models. theupper midplane, disk, where from photodestruction the very outer disk rates down are dominated to an inner by edge, stel- whichwhichlar photons is is given given (see by by Fig. the the3 location), location the photodissociation of of the the snow snow line line rate in in the the of midplane. CO midplane. in the SinceSince4000 Kthe the blackbody midplane midplane star snow snow model line line is is below essentially essentially that in controlled controlled the fiducial by by model ther- ther- to observationally locate the snow line of different molecules mal desorption (and thus by the binding energy of the particu- 3.7. Ices and snow lines malby many desorption orders (and of magnitude. thus by the A similarbinding behavior energy of occurs the particu- for the in3.7. the Ices same and disk, snow so lines that the sequence of snow lines of ices lar ice and by the dust temperature structure of the disk along Ices account for an important percentage of the matter of pro- larphotodestruction ice and by the rates dust of temperature other species. structure As a of consequence, the disk along the withIces account different for binding an important energies percentage depicted of in the Fig. matter 13 can of pro- be the midplane),+ ices appear progressively in the radial direction toplanetary disks. The high densities and cold temperatures pre- theCO/C/C midplane),interface ices shifts appear to progressively upper heights in and the radialthe atomic direction car- tracked.toplanetary disks. The high densities and cold temperatures pre- according to their binding energies. In Fig. 12, we show the vailing in the midplane regions ensure a rapid and efficient ad- accordingbon layer becomes to their wider binding (see energies. right panels In Fig. in Fig. 12, 14 we). In show general, the vailing in the midplane regions ensure a rapid and efficient ad- calculated distributions of CO and H2O ices in the T Tauri and sorption of gas-phase molecules onto dust grains, where they calculatedthe photochemically distributions active of CO layer, and where H2O forices example in the T the Tauri transi- and sorption of gas-phase molecules onto dust grains, where they Herbig Ae/Be disks. First focusing on the T Tauri disk model, we 4.settle Influence and live longof the in the stellar form spectrum of ice. The presence and the or absence Herbigtion H2 AeO/OH/Be disks. is located First focusingand the radicals on the T CTauri2H and disk CN model, are, we is settle and live long in the form of ice. The presence or absence see that CO ice, for which the adopted binding energy is 1575 of a particular type of ice in a certain disk region depends on seeshifted that to CO upper ice, layers. for which This the overall adopted shift binding of the photochemistry energy is 1575 ofphotodestruction a particular type of ice rates in a certain disk region depends on K, is only present beyond a few tens of astronomical units, while the balance between adsorption and desorption, and, because ad- K,to upper, is only less present dense, beyond and warmer a few tens layers, of astronomical together with units, thewhile lower the balance between adsorption and desorption, and, because ad- Chemicalsorption rates models are similarof protoplanetary for most molecules disks contain (see Sec. so many 2.2.5), ingre- the strength13 The snow of theline ofFUV a particular radiation species field, isdefined induce here important as the transition changes sorption rates are similar for most molecules (see Sec. 2.2.5), the 13 dientsextent to of deal each with particular the multiple type of processes ice is determined at work that by its the specific output regionin theThe where abundances snow its line gas of and a ofparticular ice several abundances species molecules. become is defined equal. For here example, as the transition stable extent of each particular type of ice is determined by its specific region where its gas and ice abundances become equal. A19, page 21 of 33 20 20 Agundez´ et al.: The chemistryA&A of 616,disks A19 around (2018) T Tauri and Herbig Ae/Be stars

10-3 fiducial T Tauri model -3 fiducial T Tauri model -3 4000 K blackbody star -3 parametric -4 10 10 10 10 4000 K blackbody star

) photodestruction rates 1 -5 parametric − 10 s photodestruction rates ( + + 10-6 CO C CO C CO C e t

a -7 -4 -4 -4

r 10 10 10 10

n -8

o 10 i t -9 C + a 10 i

c -10 C o 10 -5 -5 -5 s 10 10 10 s i 10-11 C d

o -12 t 10 o

h -13

p 10 10-6 10-6 10-6

O -14 10 abundance relative to total H C 10-15

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.15 0.20 0.25 0.15 0.20 0.25 0.15 0.20 0.25 z/r z/r z/r z/r

Fig.Fig. 14. 14.VerticalVertical structure structure of of the the CO CO photodissociation photodissociation rate rate ( (leftleft panel panel)) and and the the CO/C/C CO/C/C++ interfaceinterface ( (rightright panels panels)) at at a a radius radius of of 1 1 au au from from the the star star forfor the the T T Tauri Tauri disk. disk. We We show show results results from from our our fiducial fiducial model, model, a a model model in in which which a a 4000 4000 K K blackbody blackbody stellar stellar spectrum spectrum is is considered, considered, and and a a model model inin which which all all photodestruction photodestruction rates rates are are computed computed using using parametric parametric expressions expressions (see (see text). text). The The dotted dotted lines lines in in the the three three right right panels panels indicate indicate the the peakpeak abundance abundance of of atomic atomic carbon carbon and and the the height height at at which which it it is is reached reached in in the the fiducial fiducial model. model. The The small small drop drop in in the the abundance abundance of of CO CO seen seen around around zz//rr 00.15.15,, which which is is more more marked markedAg in inundez´ the the 4000 4000 et al.: K K blackbody The blackbody chemistry star star model, of model, disks is isaround due due to to T a aTauri maximum maximum and Herbig in in the the Aeabundance abundance/Be stars of of water. water. ∼∼

1016 H2O ice,20 which has afiducial much higherT Tauri bindingmodel energy (the adopted tionality to be fiducial50 while T Tauri Mart ´ın-Dommodel enech´ et al. (2014) quote 10 H2O ∼ C2H value) is 5773 K), is present all over the disk midplane. The fact a value of 30 (from Attard & Barnes 1998).

2 4000 K blackbody star 19 4000 K blackbody star − C2H2 15 ∼ that water10 ice extendsparametric down to the photodestruction inner disk edge rates is a particular 10 Using observations to reveal the location of snowCN lines in m parametric photodestruction rates

c 18 HCN ( 10 outcome of the T Tauri disk model adopted here, which results protoplanetary disks is still challenging, although ALMA obser-

y 14

t 10 in dusti 1 that7 is too cold to allow for efficient thermal desorption vations are starting to permit interesting constraints. For exam-

s 10 of watern ice in the inner disk midplane. Since dust tempera- ple, in the disk around the T Tauri star TW Hya the CO snow e 16 13 d 10 10 tures are strongly dependent on parameters such as the size of line has been found to lie at a radius of 20 au (Schwarz et al. n 1015 ∼ dustm grains, other T Tauri disk models may find water snow lines 2016; Zhang et al. 2017), while in the disk around the Herbig Ae

u 12 l 14 10 thato are10 further out compared to that obtained here (see, e.g., star HD 163296 it has been located at a radius of 90 au from c

Walshl et13 al. 2015). An obvious difference between the T Tauri the star (Qi et al. 2015). These two observational points are in- a 10 11 c 10 andi the Herbig Ae/Be disk models is that in the latter, snow lines t cluded in Fig. 13. While the CO snow line derived for TW Hya

r 12 shifte 1 to0 larger radii owing to the warmer dust temperatures (see is close to the value calculated in our T Tauri disk model, the v 1010 Fig. 12),1011 and as a consequence, the mass of ices in the disk be- radius derived for HD 163296 is much smaller than indicated comes smaller. / 0 1 2 by our Herbig0 Ae Be disk model,1 which puts the2 CO snow line 10 10 10 even beyond10 the outer disk1 edge.0 We however1 note0 that the co- r (au) r (au) The abundance distributions of CO and H2O ices shown in incidence in the case of TW Hya is very likely accidental be- Fig. 12 serve to illustrate two extreme cases of molecules with cause our generic T Tauri disk model is not aimed to represent Fig. 15. Calculated vertical column densities down down to to the the midplane midplane of of H H22O,O, C C22HH22,, and and HCN HCN (left (left panel) panel) and and the the radicals radicals C C2H2H and and CN CN (right (right panel) panel) asaslow a function and high of binding radius r for energies, the T Tauri respectively disk. disk. We We showshow (see Tableresults results 2). from from Other our our fiducial fiducialeither model, model, the TW a a model model Hya indisk in which which or anya a 4000 4000 other K K blackbody blackbodyparticular stellar stellar disk. spectrum Thespectrum obser- is is considered,considered,ices with binding and a model energies in which between all photodestruction those of CO rates and are H2 computed computedO have using usingvational parametric parametric finding expressions expressions of a CO snow (see (see text). text). line farther out in the HD 163296 intermediate distributions between those of these two molecules. disk than in the TW Hya disk is in line with the general expec- In general, the smaller the binding energy, the larger the radius tation that snow lines are shifted to larger radii in Herbig Ae/Be moleculesto whichT Tauri the like stars snow water are line cool, and shifts. with the simple eThisffective relationship organics temperatures C2 isH illustrated2 and of the HCN or- in sectionsCOdisks photodissociation compared compiled to in T Appendix Tauri rate as disks, afunction A. although To investigate of height taking the at into a e radiusff accountect of of becomederFig. of 13, 4000 more where K, abundant althoughwe plot at the they intermediate radius usually at which have disk an the radii important midplane compared FUV snow to using1the au poor agiven simpler statistics, by this approach, consisting parametric we haveof photodestruction just run one a object T Tauri of disk rates each model approach type, thisin theexcessline fiducial lies that for model can various greatly (see ices left a asffect panel a function the in photochemistry Fig. of 15 their), while binding of the the energies. radicals disk. whichwithresult that all may photodestruction resulting be fortuitous. from our Expandingrates fiducial are computed the model. sample through It is of seen disks the that para- seems in α γ CToThere2H evaluate and is CNa clear the experience influencetrend which a ofdecline indicates this FUV in their that excess abundances both we quantities compare (see are right our re- metricthemandatory upper expression disk to draw both in Eq.more approaches (7). definitive The yieldand conclusions. similarparameters results, It will are although alsotaken be fiducial T Tauri disk model, in which we considerq the stellar from Table B.1, while for H2 and CO we adopt unattenuated panellated in by Fig. a power 15). law of the type ED rms− , where rms stands atvery lower interesting heights, to where observationally photodestruction locate is the dominated snow line by of ISRF dif- spectrumforThe radius way of of TW in midplane Hya, which with photodestructionsnow a model line. A in∝ fit which to rates the we are data assume computed points that in ratesphotonsferent and molecules (see dust Fig. shielding3 in), the the factors same parametric disk, from so approach Heays that the et al.overestimates sequence (2017). of In snow the the maytheFig. star also 13 emits yields have as an an a exponentblackbody influenceq on atof a the temperature0.23 chemical in both of structure the 4000 T Tauri K of (both theand leftphotodissociationlines panel of ices of Fig. with 14rate di wefferentof compare CO. binding The the resulting resultingenergiesCO/C/Cdepicted CO photodissoci-+ ininterface Fig. 13 photochemicallyspectrathe Herbig are Ae shown/Be active disks. in Fig. layer This 1). ofbehavior The disks∼ lack (e.g., is of notFUV Walsh surprising excess et al. since in2012 this the). ationiscan very be rate similartracked. as a function in both ofscenarios, height at although a radius itof is 1 worth au given noting by this that Inlattermidplane our case fiducial snow results model,line in of much a we given lower have ice used photodestructionis essentially the Meudon located rates PDR at (com- the code ra- parametricthe peak abundance photodestruction of atomic rates carbon approach is lower with when that the resulting paramet- pare our values in Table B.1 with those of van Dishoeck et al. from our fiducial model. It is seen that in the upper disk both ap- todius compute at which the the photodissociation dust temperature rates in the of midplane H2 and becomes CO by solv- sim- ric photodestruction rates are used. This has some consequences 2006, or see Heays et al. 2017). The net effect is that without proaches yield similar results, although at lower heights, where ingilar the to the excitation condensation and the temperature line-by-line of FUV the ice, radiative and the transfer latter is for4.Influence the abundances of the of carbon-bearing stellar spectrum species and typically the formed FUV excess, most photodestruction rates are dominated by the photodestruction is dominated by ISRF photons (see Fig. 3), the todirectly properly proportional account for to the self binding and mutual energy shielding of the ice. effects That is, and the under the action of photochemistry, such as the radicals C2H ISRF rather than by stellar radiation. As as example, in the left parametricphotodestruction approach overestimates rates the photodissociation rate of haveobserved computed relation photodestruction between binding rates energy for a and variety midplane of species snow and CN. In fact, these two+ radicals reach abundances signifi- usingpanelline merely the of Fig. cross reflects 14 sections we show how compiled the photodissociation midplane in Appendix dust temperatureA rate. To ofinvestigate CO varies as a CO.cantlyChemical The lower resulting models in the CO of outer protoplanetary/C/C diskinterface when parametricdisks is very contain similar photodestruction so in many both ingre- sce- thefunctionwith effect radius. of of height In using Fig. at a 13 a simpler radiuswe have of approach, overplotted 1 au, where we as have both a solid run the line contribu- a T the Tauri ra- narios,ratesdients are although to used deal withcompared it is the worth multiple to noting the fiducial processes that the model atpeak work (see abundance that right the panel out- of disktionsdial profilemodel of the ofISRF in the which and dust theall temperaturephotodestruction star are taken in the into midplane rates account. are scaled computed It is upseen by atomicinput Fig. abundance carbon15). Other is distributions lower species when are can alsothe be parametric affected sensitive to photodestruction differentto many degrees.of them. throughthata factor in the the of 45. upper parametric The disk, comparison expression where photodestruction between in Eq. solid (7). The line ratesα andand aredataγ param- domi- points ratesForIn this example, are study used. while we This put hasthe the columnsome focus consequences density on the ofphotochemistry water for the does abundances not and change thus nated by stellar photons (see Fig. 3), the photodissociation rate of carbon-bearing species typically formed under the action of etersin Fig. are 13 taken suggests from that Table a factor B.1,of while proportionality for H2 and betweenCO we adopt bind- much,here we acetylene evaluate becomes the influence less abundant of a couple and of hydrogen related aspects: cyanide the is unattenuatedofing CO energy in the and rates4000 condensation andK blackbody dust shielding temperature star model factors in is thefrom below range Heays that 30-50 in et the al. is photochemistry,slightlyspectrum enhanced of the such star in and theas the rangethe radicals way of in radii whichC2H between and photodestruction CN. 1 and In fact, 10 au these rates (see (fiducialadequate2017). Inmodel for the the by left many protoplanetary panel orders of Fig. of magnitude. disks14 we modeled compare A similar here. thebehaviorFor resulting com- twoleftare radicalspanelcomputed. in reachFig. We 15 abundances concentrate). onsignificantly the T Tauri lower disk model in the for outer this occursparison, for Hollenbach the photodestruction et al. (2009) rates calculate of other this species. factor As of propor-a con- disksensitivity when parametric analysis. photodestruction rates are used compared sequence, the CO/C/C+ interface shifts to upper heights and the to the fiducial model (see right panel in Fig. 15). Other species A19,atomic page carbon 22 of 33 layer becomes wider (see right panels in Fig. 14). are also affected to different degrees. For example, while the col- In general, the photochemically active layer, where for exam- umn density of water does not change much, acetylene becomes21 ple the transition H2O/OH is located and the radicals C2H and less abundant and hydrogen cyanide is slightly enhanced in the CN are, is shifted to upper layers. This overall shift of the pho- range of radii between 1 and 10 au (see left panel in Fig. 15). tochemistry to upper, less dense, and warmer layers, together with the lower strength of the FUV radiation field, induce im- portant changes in the abundances of several molecules. For ex- 5. Conclusions ample, stable molecules like water and the simple organics C2H2 We have developed a model aimed to compute the chemical and HCN become more abundant at intermediate disk radii com- composition of a generic protoplanetary disk around a young pared to the fiducial model (see left panel in Fig. 15), while the star. The model considers a passively irradiated disk in steady radicals C2H and CN experience a decline in their abundances state and computes the physical and chemical structure of the (see right panel in Fig. 15). disk with particular attention to the disk photochemistry. In par- The way in which photodestruction rates are computed may ticular we have compiled cross sections for 29 molecules and 8 also have an influence on the chemical structure of the photo- atoms and computed the photodissociation and photoionization chemically active layer of disks (e.g., Walsh et al. 2012). In our rates at each location in the disk by solving the FUV radiative fiducial model, we have used the Meudon PDR code to compute transfer with the Meudon PDR code in a 1+1D approach. the photodissociation rates of H2 and CO by solving the excita- We have applied the model to perform a comparative tion and the line-by-line FUV radiative transfer to properly ac- study of the chemistry of disks around low-mass (T Tauri) count for self and mutual shielding effects and have computed and intermediate-mass (Herbig Ae/Be) stars. Infrared and (sub- photodestruction rates for a variety of species using the cross )millimeter observations of T Tauri and Herbig disks point to a

22 M. Agúndez et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars

5. Conclusions In any case, for HCN, C2H, and CN, the observed and calculated abundance differences between both types of disks are small, of We have developed a model aimed to compute the chemical com- a factor of a few at most. position of a generic protoplanetary disk around a young star. For other organic molecules with a certain complexity The model considers a passively irradiated disk in steady state such as CH3OH, HC3N, CH3CN, and c-C3H2, the match with and computes the physical and chemical structure of the disk observed values is not satisfactory. This fact, together with the with particular attention to the disk photochemistry. In particular ample dispersion of abundances calculated by different chem- we have compiled cross sections for 29 molecules and 8 atoms ical models in the literature, points to the chemistry of these and computed the photodissociation and photoionization rates at molecules not yet being as robust as for simpler species. In each location in the disk by solving the FUV radiative transfer addition, grain-surface chemistry, not included in our model, is with the Meudon PDR code in a 1+1D approach. likely playing an active role in the synthesis of some of these We have applied the model to perform a comparative molecules. study of the chemistry of disks around low-mass (T Tauri) A clear differentiation between T Tauri and Herbig Ae disks and intermediate-mass (Herbig Ae/Be) stars. Infrared and is found concerning ices. The warmer temperatures of Herbig Ae (sub-)millimeter observations of T Tauri and Herbig disks point disks shift snow lines to larger radii compared to T Tauri disks to a lower detection rate of molecules in the latter type of disks and as a consequence disks around intermediate-mass stars are and, for some species, somewhat lower abundances. Motivated expected to contain a substantially lower mass of ices compared by the observational studies, we have investigated the potential to T Tauri disks. chemical differentiation between disks around these two types of stars, which have very different masses and spectra. We find that Acknowledgements. We thank the anonymous referee for a detailed reading globally the chemical behavior of these two types of disk is quite and for a constructive report which helped to improve this manuscript. M.A. similar, with some important differences driven by the higher acknowledges funding support from the European Community 7th Framework Programme through a Marie Curie Intra-European Individual Fellowship (grant stellar UV flux and the warmer temperatures of Herbig Ae/Be 235753), from the European Research Council through ERC Synergy grant disks. 610256, and from Spanish MINECO through the Ramón y Cajal programme Water vapor and the simple organic molecules C2H2, HCN, (RyC-2014-16277) and grant AYA2016-75066-C2-1-P. This work was partly sup- 4 and CH4 are predicted to be very abundant ( 10− for H2O ported by the CNRS program Physique et Chimie du Milieu Interstellaire (PCMI) and a few 10 5 for the organics) in the hot inner∼ regions of co-funded by the Centre National d’Études Spatiales (CNES). We thank J. R. × − Goicoechea, E. Bron, S. Cazaux, G. M. Muñoz-Caro, R. Martín-Doménech, and disks around both T Tauri and Herbig Ae/Be stars. The main A. Fuente for useful discussions, S. 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Appendix A: Photodissociation and comprise dissociation by lines and by continuum. The photoion- photoionization cross sections ization cross section has been obtained from the theoretical study by Barsuhn & Nesbet(1978). The photoionization threshold of Photodissociation and photoionization processes are treated CH is 1170 Å (Huebner et al. 1992). using the relevant cross section as a function of wavelength. We adopt this approach for key species for which cross sec- A.2. CH+ tion data are available in the literature, excluding H2 and CO, the photodissociation of which are treated solving the excita- The photodissociation cross section of CH+ is taken from the tion and line-by-line radiative transfer and taking into account theoretical calculations of Kirby et al.(1980), where we have self-shielding effects. We are mainly interested in cross sections adopted a cross section of 10 Mb18 at the resonances lying at ranging from the Lyman cutoff, at 911.776 Å, to wavelengths in 1523, 1546, and 1579 Å. Photodissociation through the three the range 1500–4000 Å, where most molecules have their pho- excited electronic states considered by Kirby et al.(1980) , 21Σ+, todissociation threshold. We note that in protoplanetary disks, 21Π, and 31Σ+, yields C + H+ as products. stellar photons with wavelengths shorter than the Lyman cutoff may also have an impact on the photodissociation and photoion- A.3. CH2 ization rates in disk regions exposed to the star. It will be worth taking this into account in the future. The photodissociation cross section of the radical Cross sections of molecules with a closed electronic shell has been taken from van Dishoeck et al.(1996), whose calcula- have been mostly taken from experimental studies carried out tions comprise dissociation by lines and by continuum radiation. at room temperature. In the case of radicals, most of the cross Methylene has two main dissociation channels: section data come from theoretical studies. We note that cross CH + hν CH + H (A.1) sections can show a great dependence with temperature, some- 2 → C + H , (A.2) thing that may have an important impact on the chemistry of → 2 protoplanetary disks taking into account that the gas kinetic tem- but the latter is a minor one (Kroes et al. 1997), and has there- perature can take values from hundreds to thousands of degrees fore been neglected here. The ionization threshold of CH2 is Kelvin in the photon-dominated region of the disk. Of course it 1193 Å, although there are no cross section data in the literature, would be desirable to use temperature-dependent cross sections, except for the relative data in the ionization threshold region by although such data are currently limited to just a few species Litorja & Ruscic(1998). (e.g., Venot et al. 2013; McMillan et al. 2016). This will be something to take into account in the future. We have compiled photo cross sections for 29 molecules and A.4. CH3 8 atoms from original sources in the literature or from databases The UV spectra of CH3 has been recorded in photographic plates such as the MPI-Mainz UV/VIS Spectral Atlas of Gaseous 14 (Herzberg 1961; Callear & Metcalfe 1976), although no quantita- Molecules of Atmospheric Interest , which contains extensive tive measurement of the intensity was carried out. More recently, information on experimental cross sections of stable molecules, 15 the absolute photodissociation cross section was measured in the Photo Rate Coefficient Database (Huebner et al. 1992), the 2000–2400 Å region by Cameron et al.(2002) and in the the Leiden database of photodissociation and photoionization of band at 2164 Å by Khamaganov et al.(2007), in good agreement astrophysically relevant molecules16 (van Dishoeck et al. 2006; Heays et al. 2017), which contains theoretical data of numer- with previous absolute measurements: 40.1–41.2 Mb at 2136.6 Å ous radicals. It is common that experimental studies provide (Macpherson et al. 1985) and 35.1 Mb at 2164 Å (Arthur 1986). the photoabsorption cross section without disentangling whether We have adopted the photodissociation cross section measured the absorption leads to fluorescence, dissociation, or ionization. by Cameron et al.(2002) and Khamaganov et al.(2007) at Unless otherwise stated, we have assumed that absorption of wavelengths longer than 2000 Å, and have calibrated the band FUV photons not leading to ionization leads to dissociation of intensities classified in five categories, from very weak to very the molecule, that is, that the contribution of fluorescence to the strong, by Herzberg(1961) based on the absolute cross section total photoabsortion cross section is negligible. This may be a measured for the 2164 Å band. The dissociation of the methyl bad approximation at long wavelengths but is likely to be cor- radical has two possible channels: rect at short wavelengths, typically below 1500 Å. In the case of CH + hν CH + H (A.3) photoionization of atoms we have made use of databases such 3 → 2 17 CH + H , (A.4) as TOPbase, the Opacity Project atomic database (Cunto et al. → 2 1993), to retrieve cross sections for some of the atoms. In the fol- whose quantum yields are not clear. Channel (A.3) is observed lowing subsections we detail the cross section adopted for each at 2163 Å by Wilson et al.(1994), at 1933 Å by North et al. of the 29 molecules and 8 atoms considered. In Fig. A.1 we show (1995), and at 2125 Å by Wu et al.(2004), while channel the photodissociation cross section as a function of wavelength λ > for some key molecules. (A.4) is observed through fluorescence of CH at 1050 Å by Kassner & Stuhl(1994). In the absence of better constraints we A.1. CH have assumed a quantum yield of 50% for each channel. As concerns photoionization, we adopt the absolute cross section The photodissociation cross section of methylidyne has been measured by Gans et al.(2010) from the photoionization thresh- taken from the calculations of van Dishoeck(1987), which old (1260 Å) to 1085 Å, and consider a constant cross section shortward of 1085 Å. The adopted cross section is, within uncer- 14 http://www.uv-vis-spectral-atlas-mainz.org tainties, similar to those reported in previous studies (Taatjes 15 http://phidrates.space.swri.edu/ et al. 2008; Loison 2010). 16 http://home.strw.leidenuniv.nl/$\sim$ewine/photo/ 17 18 18 2 http://cdsweb.u-strasbg.fr/topbase/topbase.html 1 Mb is equal to 10− cm . A19, page 27 of 33 Agundez´ et al.: The chemistryA&A of disks 616, aroundA19 (2018) T Tauri and Herbig Ae/Be stars

10-15 10-15 ) 2 H2O OH m

c CH4 HCN (

n NH3 SO2

o -16 -16 i N t 10 2 10 CN c

e C2H2 H2S s CO HCl s 2 s o

r -17 -17 c 10 10

n o i t a i c

o -18 -18 s 10 10 s i d o t o h p 10-19 10-19 1000 1200 1400 1600 1800 2000 1000 1200 1400 1600 1800 2000 λ (Å) λ (Å)

Fig.Fig. A.1. A.1.PhotodissociationPhotodissociation cross cross section section of of various various molecules molecules as as a a function function of of wavelength. wavelength. The The position position of of the the Ly- Ly-ααlineline at at 1216 1216 Å is marked with aa vertical vertical dashed dashed line. line.

(1260A.5. CH Å)4 to 1085 Å, and consider a constant cross section short- A.8.A.9. COH2H2 ward of 1085 Å. The adopted cross section is, within uncertain- In the case of methane the photoabsorption cross section has ties, similar to those reported in previous studies (Taatjes et al. ForThe acetylene photoabsorption the photoionization cross section cross of the section hydroxyl at wavelengths radical has been taken from the experimental study by Au et al.(1993) 2008; Loison 2010). shorterbeen measured than the ionization by Nee & threshold Lee(1984 of) in 1087.6 the wavelength Å has been range taken while the photoionization cross section (the ionization threshold from1150–1830 the compilationÅ. On the theoretical by Hudson side, (1971). the photodissociation We have subtracted has of CH4 is 983.2 Å) has been measured by Wang et al.(2008). thealso photoionization been studied crossby van section Dishoeck from &the Dalgarno photoabsorption(1984). crossThe Methane has various photodissociation channels leading to the A.5. CH4 sectionexperimental measured values by Cooper are somewhat et al. (1995) higher to obtainthan the the theoretical photodis- radicals CH , CH , and CH: 3 2 sociationones, although cross section, calculations which indicate is assumed that to photodissociation yield the C2H radi- by lines is also important at wavelengths shorter than 1150 Å. InCH the+ casehν ofCH methane+ H the photoabsorption cross section(A.5) has cal with a 100 % efficiency. been4 taken→ from3 the experimental study by Au et al. (1993) We adopt the theoretical photodissociation cross section, mainly CH (X3B ) + H + H or CH (a1A ) + H , (A.6) while the photoionization→ 2 1 cross section (the2 ionization1 2 threshold because it spans over a broader wavelength range. Relative pho- CH + H + H or CH + H + H + H, (A.7) A.9.toionization OH cross section has been measured by Dehmer(1984) of CH4 is 983.2→ Å) has2 been measured by Wang et al. (2008). up to the threshold at 952.5 Å (Lide 2009). The absolute scale of Methane has various photodissociation channels leading to the The photoabsorption cross section of the hydroxyl radical has whose quantum yields have been measured by Gans et al.(2011) the photoionization cross section is fixed by setting a value of 2.5 radicals CH3, CH2, and CH: been measured by Nee & Lee (1984) in the wavelength range at 1182 and 1216 Å. Mb at 946 Å, according to the calculations of Stephens & McKoy 1150-1830 Å. On the theoretical side, the photodissociation has (1988). also been studied by van Dishoeck & Dalgarno (1984). The ex- CHA.6.4 + Ch2ν CH3 + H (A.5) → 3 1 perimental values are somewhat higher than the theoretical ones, CH2(X B1) + H + H or CH2(a A1) + H2, (A.6) The photodissociation→ and photoionization cross sections of C2 althoughA.10. H2O calculations indicate that photodissociation by lines is have been takenCH + fromH2 + theH theoretical or CH + H studies+ H + H by, Pouilly(A.7) et al. also important at wavelengths shorter than 1150 Å. We adopt → The photoabsorption cross section of water has been taken from (1983) and Toffoli & Lucchese(2004), respectively. The pho- the theoretical photodissociation cross section, mainly because various experimental studies: Fillion et al.(2004) for the wave- toionization threshold of C2 is 1020 Å (Toffoli & Lucchese it spans over a broader wavelength range. Relative photoioniza- whose quantum yields have been measured by Gans et al. (2011) 2004). tionlength cross range section 999–1139 has beenÅ, measuredMota et al. by(2005 Dehmer) in the (1984) wavelength up to the at 1182 Å and 1216 Å. thresholdrange 1148–1939 at 952.5 Å, (Lide and Chan 2009). et The al.( absolute1993a) elsewhere scale of the in pho- the range from the Lyman cutoff to 2060.5 Å. The photoionization A.7. C2H toionization cross section is fixed by setting a value of 2.5 Mb atcross 946 section Å, according for wavelengths to the calculations shorter than of the Stephens ionization & McKoy thresh- A.6.In the C2 case of the we adopt the photodissociation (1988).old of H2O (982.4 Å) has been taken from the experimental study cross section by lines calculated by van Hemert & van Dishoeck of Fillion et al.(2003). Water has two main photodissociation The photodissociation and photoionization cross sections of C (2008). The main dissociation channel yields C2 + H (Duflot2 channels, yielding OH radicals and O atoms: have been taken from the theoretical studies by Pouilly et al. et al. 1994; Sorkhabi et al. 1997; Mebel et al. 2001; Apaydin et al. A.10. H O (1983) and Toffoli & Lucchese (2004), respectively. The pho- 2 2004). The photoionization threshold of C2H is 1068 Å (Lide ff H2O + hν OH + H, (A.8) toionization2009), although threshold to our knowledge of C2 is 1020 there are Å not (To crossoli & section Lucchese data The photoabsorption→ cross section of water has been taken from 2004). 3 1 available in the literature. various experimentalO( P) + studies:H + H Fillion or O( etD) al.+ H (2004)2, for the wave-(A.9) length range→ 999-1139 Å, Mota et al. (2005) in the wavelength

A.8. C2H2 range 1148-1939 Å, and Chan et al. (1993a) elsewhere in the A.7. C2H where channel (A.8) dominates over channel (A.9). Water can range from the Lyman cutoff to 2060.5 Å.˜ 1 The photoionization˜1 For acetylene the photoionization cross section at wavelengths be photodissociated in the first band X A1 A B1 at wave- cross section for wavelengths shorter than the ionization−˜ 1 ˜ 1 thresh- Inshorter the case than of the theionization ethynyl radical threshold we adopt of 1087.6 the photodissociationÅ has been taken lengths longer than 1360 Å and in the second X A1 B A1 and cross section by lines calculated by van Hemert & van Dishoeck oldhigher of H bands2O (982.4 at shorter Å) has wavelengths. been taken Quantum from the yields experimental− of 78% from the compilation by Hudson(1971). We have subtracted study of Fillion et al. (2003). Water has two main photodisso- (2008). The main dissociation channel yields C2 + H and 22% for channels (A.8) and (A.9) have been measured by the photoionization cross section from the photoabsorption cross ciation channels, yielding OH radicals and O atoms: (Duflotsection et measured al. 1994; by Sorkhabi Cooper et et al. al.( 1997;1995) to Mebel obtain et al. the 2001; pho- Slanger & Black(1982) at 1216 Å (Lyα). We adopt these values Apaydintodissociation et al. 2004). cross section, The photoionization which is assumed threshold to yield of the C2H C isH for λ < 1360 Å while at longer wavelengths we take quantum 2 + hν + , 1068radical Å with(Lide a 2009), 100% efficiency. although to our knowledge there are not Hyields2O of 99%OH and 1%H (Crovisier 1989). (A.8) → 3 1 cross section data available in the literature. O( P) + H + H or O( D) + H2, (A.9) A19, page 28 of 33 →

28 M. Agúndez et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars

A.11. O2 1994; Vetter et al. 1996). As concerns photoionization, we used the relative cross section measured by Gibson et al.(1985) in In the wavelength range of photoionization of molecular the wavelength range 745–1125 Å (the ionization threshold is (up to the ionization threshold of 1027.8 Å, e.g., Huebner et al. 1130 Å) and fixed the absolute scale by assuming a guess value 1992), the photoabsorption and photoionization cross sections have been taken from the experimental study of Holland et al. of 10 Mb for the cross section at 912 Å. (1993). Up to 1750 Å, the photoabsorption cross section has been taken from Yoshino et al.(2005), and from Chan et al.(1993b) at A.16. NH3 longer wavelengths. The dissociation threshold of O2 is 2423.7 Å The photoabsorption cross section of is taken from (Huebner et al. 1992), although at wavelengths longer than Edvardsson et al.(1999) up to the ionization threshold of 1231 Å, 1850 Å the photodissociation cross section already becomes ∼ from Wu et al.(2007) in the wavelength range 1231–1440 Å, negligible. and from Chen et al.(2006a) at longer wavelengths. The data measured by Burton et al.(1993) at a lower spectral resolution A.12. H2CO is consistent with the data described above. The photoioniza- tion cross section has been taken from Edvardsson et al.(1999). The photoabsorption cross section of formaldehyde is taken As concerns photodissociation, there are different channels for from measurements by Cooper et al.(1996). The cross section ammonia, leading to NH and NH radicals, of photoionization, which occurs at wavelengths shorter than 2 1400 Å, has been measured by Mentall et al.(1971). The ∼ NH3 + hν NH2 + H, (A.15) photodissociation channels of H2CO are → 3 1 NH(X Σ−) + H + H or NH(a ∆) + H , (A.16) → 2 H CO + hν CO + H , (A.10) 2 → 2 where channel (A.15) dominates over the production of NH with CO + H + H, (A.11) → a yield 0.694. The quantum yields over the wavelength range HCO + H, (A.12) of interest≥ have been taken from various measurements carried → out at different wavelengths (McNesby et al. 1962; Okabe et al. where channel (A.12) can be neglected as it occurs in the 1967; Groth et al. 1968; Lilly et al. 1973; Slanger & Black 1982). wavelength range 2000–3340 Å, where the absorption cross section of H2CO is three orders of magnitude lower than at A.17. N2 λ < 2000 Å. Channels (A.10) and (A.11) occur with similar quantum yields (Stief et al. 1972), however, since the implica- Molecular is ionized by photons with energies higher tions of producing either molecular or atomic hydrogen in the than 15.58 eV and thus only photodissociation, and not pho- toionization, occurs in our wavelength range of interest. The photodissociation of H2CO are small for the chemistry of pro- toplanetary disks, we have assumed that only channel (A.10) photoabsorption cross section has been taken from the experi- occurs. ments carried out by Chan et al.(1993c). The photodissociation threshold of N2 is 1270.85 Å, although in the practice only photons with λ < 1000 Å are efficient enough in dissociating A.13. CO2 molecular nitrogen. The ionization threshold of CO2 is 900 Å and thus photoion- ization does not occur in our wavelength range of interest. A.18. CN The adopted photoabsorption cross section is based on the critical evaluation by Huestis & Berkowitz(2010). The only The photodissociation cross section of the CN radical has been allowed channel in the photodissociation of carbon dioxide is calculated by Lavendy et al.(1987). The photodissociation that yielding CO + O (see, e.g., Huebner et al. 1992). threshold is 1600 Å (Huebner et al. 1992), although in the prac- tice the cross section becomes vanishingly small at wavelengths A.14. NH longer than 1100 Å. Since the ionization threshold of CN is ∼ 911.756 Å (Lide 2009), slightly shorter than that of hydrogen, In the case of the NH radical the photodissociation cross section we do not consider photoionization here. has been calculated by Kirby & Goldfield(1991). The photoion- ization cross section has been calculated by Wang et al.(1990) to A.19. HCN be around 8 Mb in the narrow wavelength range from the Lyman cutoff to the ionization threshold at 919.1 Å. Hydrogen is ionized by photons with wavelengths shorter than the Lyman cutoff and thus photoionization of HCN is not con-

A.15. NH2 sidered here. We have adopted the photoabsorption cross section measured by Nuth & Glicker(1982) up to λ = 1469.2 Å. We The photodissociation cross section of NH2 is taken from the assume that at longer wavelengths the contribution to photodis- theoretical calculations of Koch(1997). There are two possible sociation is small and that the main photodissociation channel dissociation channels: leads to the CN radical. NH + hν NH + H, (A.13) 2 → A.20. HNC N + H2, (A.14) → The photoabsorption cross section of HNC has recently been where only channel (A.13) is considered here, based on exper- calculated simultaneously with that of the most stable isomer imental and theoretical evidence that the major channel in the HCN by Chenel et al.(2016) in the 7–10 eV energy range. More photodissociation of NH2 leads to NH radicals (Biehl et al. recently, Aguado et al.(2017) have extended those calculations

A19, page 29 of 33 A&A 616, A19 (2018) including higher electronic states to cover the 7–13.6 eV energy in addition to some oscillator strength guesses. For the pho- range. We adopted the cross section calculated by Aguado et al. toionization cross section, we used the relative measurements (2017) and assumed that absorption in the studied wavelength carried out by Norwood et al.(1991) in the wavelength range range leads mainly to dissociation rather than fluorescence. The 1000–1100 Å (the ionization threshold is 1095.5 Å) and fixed the ionization threshold of HNC is 992 Å (Lide 2009), although the absolute scale by assuming a guess value of 10 Mb for the cross relevant cross section is not known. section at 1000 Å. ≤ A.21. NO A.26. SO The photoabsorption cross section of was taken from For , the cross section data available in the Watanabe et al.(1967) up to 1350 Å and from Guest & Lee literature cover just a limited spectral region. The photoabsorp- (1981) at longer wavelengths. The photoionization cross section tion cross section has been measured in the wavelength ranges was measured by Watanabe et al.(1967) up to the ionization 1150–1350 Å and 1900–2350 Å by Nee & Lee(1986) and by threshold, at 1340 Å. The photodissociation cross section was Phillips(1981), respectively. In the gap between these two spec- obtained by subtracting the cross sections due to photoionization tral regions and at wavelengths shorter than 1150 Å we have (Watanabe et al. 1967) and to fluorescence (as characterized by adopted an arbitrary photodissociation cross section of 5 Mb. Guest & Lee 1981) from the total photoabsorption cross section. The ionization threshold of SO is 1205 Å (Huebner et al. 1992). As photoionization cross section we used the photoelectron spec- A.22. SH trum measured by Norwood & Ng(1989) in the wavelength In the case of the mercapto radical, various theoretical stud- range 1025–1225 Å and scaled it assuming a cross section of 10 Mb at 1025 Å. ies have investigated the photodissociation dynamics of selected ≤ electronic states, mostly the first excited state A2Σ+ but also 2 2 2 A.27. SO higher ones such as Σ−, ∆, and 2 Π (e.g., Wheeler et al. 1997; 2 Chen et al. 2006b; Janssen et al. 2007). However, no quantitative The photoabsorption cross section of is based on measurement of the photodissociation cross section across the various experimental studies dealing with different wavelength FUV range is available. We have therefore adopted the photodis- ranges: Holland et al.(1995) up to the ionization threshold of sociation cross section from the Leiden database, whose data are SO2 at 1004 Å, Feng et al.(1999c) in the range 1004–1061 Å, largely based on the calculations of Bruna & Hirsch(1987). The Manatt & Lane(1993) in the range 1061–1717.7 Å, and Wu et al. photoionization threshold of SH is 1190 Å, although there are no (2000) at longer wavelengths. The photoionization cross section data on the relevant cross section. is taken from Holland et al.(1995). The photodissociation of SO 2 has two possible channels: A.23. SH+ SO2 + hν SO + O, (A.19) For the SH+ ion, the photodissociation cross section was taken → S + O2, (A.20) from the Leiden database, where the recent calculations by → McMillan et al.(2016) were used adopting the cross section of with quantum yields of 50% for each channel, as measured by photodissociation from the ground v = 0 and J = 0 level (see Driscoll & Warneck(1968) at 1849 Å. We adopt these quantum Heays et al. 2017). Photodissociation at FUV wavelengths is yields from the Lyman cutoff to the photodissociation threshold dominated by transitions involving the excited electronic states of channel (A.20), lying at 2070 Å (Huebner et al. 1992), and 3 3 + 3 Σ− and 3 Π, which yield S + H as products (McMillan et al. assume that only channel (A.19) occurs from 2070 to 2179 Å, 2016). this latter value being the photodissociation threshold of the channel leading to SO (Huebner et al. 1992). A.24. H2S For hydrogen sulfide the photoabsorption cross section has been A.28. HF taken from Feng et al.(1999a) and the photoionization yield from Hydrogen fluoride is ionized by photons with energies above Feng et al.(1999b). We just consider the dissociation channel 16 eV and thus only photodissociation is considered here. The leading to SH + H, which dominates over the others (see, e.g., photodissociation cross section of HF was obtained from the the- Cook et al. 2001). There are two possible ionization channels oretical calculations by Li et al.(2010), which are in good agree- (Huebner et al. 1992): ment with the experimental values measured in the wavelength + range 1070–1450 Å by Nee et al.(1985). The photodissociation H S + hν H S + e−, (A.17) 2 → 2 cross section of HF has the shape of a continuous band centered + + + , S H2 e− (A.18) at 1230 Å, which corresponds to the X1Σ+ a1Π transition. → ∼ − where channel (A.17) has a threshold of 1185.25 Å and dom- A.29. HCl inates, while channel (A.18) has a threshold of 927 Å and accounts for just 5% of the total ionization quantum yield. We The photoabsorption cross section of has been + therefore consider that ionization yields H2S in all cases. measured by Brion et al.(2005). The photoionization yield up to the ionization threshold of HCl, at 972.5 Å, has been taken from A.25. CS the study of Daviel et al.(1984).

In the case of carbon monosulfide, the photodissociation cross A.30. Photoionization of atoms section was taken from the Leiden database, whose data are based on absorption lines measurements by Stark et al.(1987) We have also taken into account the photoionization cross and vertical excitation energies calculated by Bruna et al.(1975), sections of those atoms which can be ionized by photons

A19, page 30 of 33 AgM.undez´ Agúndez et al.: et al.: The The chemistry chemistry of disksof disks around around T Tauri T Tauri and and Herbig Herbig Ae Ae/Be/Be stars stars todissociationwith wavelengths cross longer section than of HF 911.776 was obtainedÅ. Among from the the elements theo- 10-15 retical calculations by Li et al. (2010), which are in good agree- 107 included in the chemical network those atoms are, in order CO2 6 mentof increasing with the experimental photoionization values threshold measured (taken in thefrom wavelength Lide 2009 10-16 10

range 1070-1450 Å by Nee et al. (1985). The photodissociation H2O 5 ) and given in parentheses): Cl (956.11 Å), C (1101.07 Å), 10 1 ) − 2 -17 cross section of HF has the shape of a continuous band centered Herbig Ae/Be Å 10 P (1182.30 Å), S (1196.76 Å), Si (1520.97 Å), Fe (1568.9 Å), m 4 1

+ c 10 1 1 − (

at 1230 Å, which corresponds to the X Σ a Π transition. s

Mg (1621.51 Å), and Na (2412.58 Å). Data for chlorine have been n

3 2

∼ − o i -18 10 − taken from the measurements by Ruscic & Berkowitz(1983). t 10 c m e

2 c s

Cross sections for sulfur and sodium were taken from the TOP- 10 g A.29. HCl s r s -19 T Tauri e

base database, while for phosphorus, carbon, and silicon we o 1

10 ( r

10

19 c adopted the analytic fits by Verner et al.(1996) . For magnesium λ

The photoabsorption cross section of hydrogen chloride has been F 100 and iron we used the cross sections in the Leiden database. -20 5 measured by Brion et al. (2005). The photoionization yield up to 10 interstellar 10 × the ionization threshold of HCl, at 972.5 Å, has been taken from 10-1 the study of Daviel et al. (1984). 10-21 10-2 Appendix B: Photodissociation and 1000 1200 1400 1600 1800 photoionization rates λ (Å) A.30. Photoionization of atoms The photodissociation and photoionization rate of a given Fig.Fig. B.1. B.1.FUVFUV photodissociation photodissociation cross cross sections sections of of H H22OO and and CO CO22 super-super- Wespecies have also depends taken on into the account relevant the cross photoionization section and the cross strength sec- imposedimposed on on the the FUV FUV spectrum spectrum of of the the T T Tauri Tauri and and Herbig Herbig Ae Ae/Be/Be stars tionsand of spectral those atoms shape which of the canFUV be radiation ionized by field. photons In protoplanetary with wave- (flux(flux at at 1 1 au) au) and and on on the the spectral spectral shape shape of of the the interstellar interstellar radiation radiation lengthsdisks there longer are than two 911.776main sources Å. Among of FUV the radiation, elements the included interstel- field.field. We We note note for for example example that that Ly Lyααradiationradiation becomes becomes very very important important inlar the radiation chemical field network (ISRF) those and the atoms star, are, each in one order having of increasing a different toto photodissociate photodissociate H H22OO but but not not CO CO22,, which which is is photodissociated photodissociated more more effiefficientlyciently at at wavelengths wavelengths shorter shorter than than 11501150 Å.Å. photoionizationstrength, spectral threshold shape, and(taken illumination from Lide geometry. 2009 and Therefore, given in ∼∼ parentheses):the relative contributionCl (956.11 Å), of eachC (1101.07 field to Å), the P various (1182.30 photopro- Å), S (1196.76cesses can Å), be Si quite (1520.97 different Å), depending Fe (1568.9 on Å), the Mg particular (1621.51 process Å), where the term in parentheses is the photon flux per unit time, (via the spectral shape of the cross section) and the position in Hyaarea, spectrum and wavelength, but not by and a the blackbody integral at extends 4000 K, from the the photodis- Lyman and Na (2412.58 Å). Data for chlorine have been taken from the sociation and photoionization efficiency of the T Tauri radiation the disk (via the exposure to each radiation field). cutoff (912 Å) to a maximum wavelength λ , which depends measurements by Ruscic & Berkowitz (1983). Cross sections for max The effect of the different spectral shapes of interstellar fieldon each is greatly process. enhanced with respect to the blackbody assump- sulfur and sodium were taken from the TOPbase database, while and stellar radiation fields on the photoprocesses occurring in tion.In order to evaluate how the rates of the various photopro- for phosphorus, carbon, and silicon we adopted the analytic fits protoplanetary disks has been investigated by van Dishoeck cessesFollowing vary with the idea the ofdepth van intoDishoeck the disk, et al. we (2006), made here use we of arethe 19 ff ff byet Verner al.(2006 et). al. These (1996) authors. For approximated magnesium and the iron stellar we emission used the interestedMeudonPDR in evaluating code (Le the Petit e ect et ofal. the2006 di)erent to compute spectral the shapes pho- cross sections in the Leiden database. of interstellar and stellar radiation fields (see, e.g., Fig. B.1) of T Tauri and Herbig Ae stars as blackbodies at temperatures of todissociation and photoionization rates as a function of AV . We 4000 K and 10 000 K, respectively. One of the most dramatic onconsider the photodissociation a plane-parallel and cloud photoionization illuminated on rates one of side assorted by an effects found is that the 4000 K blackbody field is much less species.external To radiation that purpose field we corresponding scaled the T Tauri to either and theHerbig ISRF, Ae/ theBe Appendixefficient in B: photodissociating Photodissociation and photoionizing and molecules than radiationT Tauri star, fields or to the get Herbig the same Ae/Be energy star, density where of the the stellar ISRF fields over thephotoionization interstellar and 10 000 rates K blackbody fields. The reason is that thehave wavelength been scaled range to the 912-2400 FUV energy Å, which density amounts of theISRF. to 1.07 The 13 3 × the 4000 K blackbody emits little at short wavelengths, where 10cloud− erg has cm uniform− adopting density the of ISRF H nuclei of Draine and gas (1978) kinetic and tempera- the ex- The photodissociation and photoionization rate of a given 4 8 3 dissociation and ionization take place. A similar study using pressionsture. We givenverified in that Sect. densities 2.1.1. This in the is range almost 10 twice10 thecm− valueand species depends on the relevant cross section and the strength 14 3 an updated set of cross sections has been recently carried out giventemperatures by Habing ranging (1968), from 5.6 10010 to− 1000erg K, cm typical− . For− values a given in thera- and spectral shape of the FUV radiation field. In protoplanetary × by Heays et al.(2017). Here we have adopted as proxies of the diationphoton field dominated characterized region ofby protoplanetary a specific intensity disks,Iλ yieldand aidenti- pho- disks there are two main sources of FUV radiation, the interstel- T Tauri and Herbig Ae/Be radiation fields the spectra of TW Hya toprocesscal results. characterized The resulting by photodissociation a cross section σ andλ, the photoionization rate Γ can be ff larand radiation AB Aurigae field (ISRF) described and in the Sect. star, 2.1.1 each and one shown having ina Fig. di1erent. As a computedrates as a as function of the visual extinction have been fitted in strength,consequence spectral of the shape, strong and FUV illumination excess and geometry. Lyα emission Therefore, usually the range A = 0–5 according to the standard expression given λmax V thepresent relative in contributionT Tauri stars, of which each is field accounted to the forvarious by the photopro- TW Hya by Eq. (7). Weλ use this expression for simplicity, although we Γ = 4π Iλ σλdλ, (B.1) cessesspectrum can be but quite not bydiff aerent blackbody depending at 4000 on the K, particular the photodissocia- process note that morehc accurate fits can be obtained with more elaborate Z912 (viation the and spectral photoionization shape of theefficiency cross section) of the T and Tauri the radiation position field in expressions, for example, considering two coefficients instead of theis disk greatly (via enhanced the exposure with torespect each toradiation the blackbody field). assumption. whereone in the the term exponential in parentheses term (Roberge is the etphoton al. 1991 flux) or per involving unit time, the The effect of the different spectral shapes of interstellar and area, and wavelength, and the integral extends from the Lyman Following the idea of van Dishoeck et al.(2006), here we are 2nd-order exponential integral E2 (Neufeld et al. 2009; Roueff stellarinterested radiation in evaluating fields on the the effect photoprocesses of the different occurring spectral in shapes pro- cutoet al.ff 2014(912; Å)Heays to a et maximum al. 2017). wavelength λmax, which depends toplanetaryof interstellar disks and has stellar been radiationinvestigated fields by(see, van Dishoeck e.g., Fig. et B.1 al.) on eachIn Table process. B.1 we list unattenuated rates α and attenuation fac- (2006).on the These photodissociation authors approximated and photoionization the stellar rates emission of assorted of torsInγ orderunder to different evaluate radiation how the fields rates offor the the various photodissociation photopro- Tspecies. Tauri and To Herbig that purpose Ae stars we scaledas blackbodies the T Tauri at and temperatures Herbig Ae/Be of cessesand photoionization vary with the processesdepth into involving the disk, the we 29 made molecules use of and the 4000radiation K and fields 10,000 to K, get respectively. the same energy One of density the most ofdramatic the ISRF Meudon8 atoms PDRdiscussed code in (Le Appendix Petit etA al.20 2006). If we to focus compute on the the unatten- pho- effoverects thefound wavelength is that the range 4000 912–2400 K blackbodyÅ,field which is amounts much less to todissociationuated rates α, and we see photoionization that the rates calculated rates as a under function the T of TauriAV . 13 3 effi1.07cient in10− photodissociatingerg cm− adopting and photoionizingthe ISRF of Draine molecules(1978) than and Weradiation consider field a plane-parallelare not very different cloud illuminated from those computed on one side under by thethe interstellar expressions× and given 10,000 in K Sect. blackbody 2.1.1. Thisfields. is The almost reason twice is that the anthe external ISRF (within radiation one field order corresponding of magnitude), to eitherwhile inthe the ISRF, case the of the 4000 K blackbody emits little at short14 wavelengths,3 where T Tauri star, or the Herbig Ae/Be star, where the stellar fields value given by Habing(1968), 5.6 10− erg cm− . For a given 20 dissociationradiation field and characterized ionization take by a place. specific× A intensity similarI studyand usinga pho- haveThe been interstellar scaled unattenuated to the FUV rates energyα in Table density B.1 ofhave the been ISRF. scaled The up λ by a factor of two with respect to the output values from the one-side antoprocess updated characterized set of cross sections by a cross has section been recentlyσλ, the rate carriedΓ can out be cloud has uniform density of H nuclei and gas kinetic temper- by Heays et al. (2017). Here we have adopted as proxies of the ature.illuminated We verified clouds modeled that densities with the in Meudon the range PDR 10 code.4-10 This8 cm way,3 and our computed as tabulated values are in line with those in the literature (Roberge− et al. T Tauri and Herbig Ae/Be radiation fields the spectra of TW Hya temperatures ranging from 100 to 1000 K, typical values in the λmax 1991; van Dishoeck et al. 2006; Heays et al. 2017) and in the astrochem- and AB Aurigaeλ described in Sect. 2.1.1 and shown in Fig. 1. As photonical databases dominatedUMIST regionand KIDA of. protoplanetary We note, however, disks, that when yield modeling identi- Γ = 4π Iλ σλdλ, (B.1) a consequence912 ofhc the strong FUV excess and Lyα emission usu- calprotoplanetary results. The disks resulting or clouds photodissociation that are illuminated and on photoionization just one side, the ally presentZ in T Tauri stars, which is accounted for by the TW ratesother as being a function optically of thick, the one visual must extinction use half the have unattenuated been fitted rates inα 19  http://www.pa.uky.edu/~verner/photo.html thelisted range in TableAV =B.10-5. according to the standard expression given 19 http://www.pa.uky.edu/ verner/photo.html by Eq. (7). We use this expression for simplicity, although we ∼ A19, page 31 of 33

31 A&A 616, A19 (2018)

Table B.1. Photodissociation and photoionization rates for various radiation fields.

Interstellara T Taurib Herbig Ae/Bec 1 1 1 Reaction α (s− ) γ α (s− ) γ α (s− ) γ CH + hν C + H 9.8 10 10 1.58 9.9 10 10 1.02 2.5 10 9 0.89 → × − × − × − CH + hν CH+ + e 9.3 10 10 3.20 1.6 10 10 2.71 1.1 10 11 2.64 → − × − × − × − CH+ + hν C + H+ 3.3 10 10 3.02 1.0 10 10 2.39 2.2 10 11 1.89 → × − × − × − CH + hν CH + H 6.5 10 10 1.89 3.3 10 10 1.45 9.4 10 10 1.42 2 → × − × − × − CH + hν CH + H 3.6 10 10 2.06 2.0 10 10 1.55 4.8 10 10 1.54 3 → 2 × − × − × − CH + hν CH + H 3.6 10 10 2.06 2.0 10 10 1.55 4.8 10 10 1.54 3 → 2 × − × − × − CH + hν CH+ + e 3.3 10 10 2.91 5.4 10 10 2.23 8.1 10 12 2.32 3 → 3 − × − × − × − CH + hν CH + H 4.7 10 10 2.75 7.8 10 10 2.22 5.3 10 11 2.07 4 → 3 × − × − × − CH + hν CH + H 8.8 10 10 2.83 1.0 10 9 2.23 7.0 10 11 2.09 4 → 2 2 × − × − × − CH + hν CH + H + H 1.2 10 10 2.83 1.4 10 10 2.23 9.7 10 12 2.09 4 → 2 × − × − × − CH + hν CH+ + e 1.4 10 11 3.95 1.8 10 12 3.36 1.4 10 13 3.37 4 → 4 − × − × − × − C + hν C + C 1.3 10 10 2.97 3.9 10 11 2.52 3.4 10 12 2.45 2 → × − × − × − C + hν C+ + e 2.1 10 10 3.82 3.9 10 11 3.23 2.3 10 12 3.22 2 → 2 − × − × − × − C H + hν C + H 1.6 10 9 2.48 1.8 10 9 2.13 5.6 10 10 1.83 2 → 2 × − × − × − C H + hν C H + H 4.4 10 9 2.46 4.2 10 9 2.08 1.9 10 9 1.81 2 2 → 2 × − × − × − C H + hν C H+ + e 3.3 10 10 3.37 9.0 10 11 2.87 6.0 10 12 2.87 2 2 → 2 2 − × − × − × − OH + hν O + H 3.8 10 10 2.33 5.1 10 10 1.99 1.7 10 10 1.62 → × − × − × − OH + hν OH+ + e 5.2 10 12 3.95 6.5 10 13 3.35 5.5 10 14 3.37 → − × − × − × − H O + hν OH + H 6.8 10 10 2.22 1.4 10 9 2.01 3.4 10 10 1.52 2 → × − × − × − H O + hν O + H 1.0 10 10 2.70 3.5 10 10 2.21 9.5 10 12 1.74 2 → 2 × − × − × − H O + hν H O+ + e 2.7 10 11 3.84 6.2 10 12 3.22 3.6 10 13 3.23 2 → 2 − × − × − × − O + hν O + O 7.3 10 10 2.31 3.5 10 10 1.81 5.6 10 10 1.77 2 → × − × − × − O + hν O+ + e 5.1 10 11 3.72 1.2 10 11 3.08 8.6 10 13 3.08 2 → 2 − × − × − × − H CO + hν CO + H 1.6 10 9 2.16 2.5 10 9 1.80 1.0 10 9 1.37 2 → 2 × − × − × − H CO + hν H CO+ + e 4.3 10 10 3.22 1.0 10 10 2.76 7.6 10 12 2.69 2 → 2 − × − × − × − CO + hν CO + O 1.1 10 9 3.01 2.4 10 10 2.41 4.3 10 11 1.93 2 → × − × − × − NH + hν N + H 4.8 10 10 2.46 2.0 10 10 2.00 2.7 10 10 1.94 → × − × − × − NH + hν NH+ + e 1.8 10 12 4.00 7.4 10 15 3.41 1.7 10 14 3.42 → − × − × − × − NH + hν NH + H 8.9 10 10 1.92 4.2 10 10 1.50 1.1 10 9 1.47 2 → × − × − × − NH + hν NH+ + e 1.1 10 10 3.44 3.1 10 11 2.92 2.1 10 12 2.91 2 → 2 − × − × − × − NH + hν NH + H 1.2 10 9 1.99 6.9 10 10 1.58 1.2 10 9 1.41 3 → 2 × − × − × − NH + hν NH + H 3.1 10 10 2.72 1.1 10 9 2.21 3.6 10 11 2.03 3 → 2 × − × − × − NH + hν NH+ + e 4.2 10 10 3.04 1.8 10 10 2.34 8.2 10 12 2.50 3 → 3 − × − × − × − N + hν N + N 3.6 10 10 3.81 7.6 10 11 3.21 3.9 10 12 3.19 2 → × − × − × − CN + hν C + N 1.0 10 9 3.55 1.8 10 10 3.02 1.2 10 11 3.04 → × − × − × − HCN + hν CN + H 1.9 10 9 2.82 4.5 10 9 2.22 1.6 10 10 2.01 → × − × − × − HNC + hν CN + H 9.4 10 10 2.45 3.2 10 9 2.16 3.9 10 10 1.80 → × − × − × − NO + hν N + O 4.7 10 10 2.01 2.3 10 10 1.60 4.1 10 10 1.41 → × − × − × − NO + hν NO+ + e 2.6 10 10 3.00 2.4 10 10 2.27 8.7 10 12 2.24 → − × − × − × − SH + hν S + H 1.3 10 9 1.89 2.0 10 9 1.54 1.8 10 9 1.30 → × − × − × − SH+ + hν S + H+ 6.9 10 10 1.89 3.2 10 10 1.05 3.9 10 10 0.80 → × − × − × − H S + hν SH + H 3.2 10 9 2.26 3.4 10 9 1.91 1.8 10 9 1.58 2 → × − × − × − H S + hν H S+ + e 7.2 10 10 3.15 1.7 10 10 2.68 1.3 10 11 2.61 2 → 2 − × − × − × − CS + hν C + S 9.5 10 10 2.60 4.2 10 9 2.20 1.7 10 10 1.98 → × − × − × − CS + hν CS+ + e 1.7 10 10 3.30 4.1 10 11 2.81 2.8 10 12 2.81 → − × − × − × − SO + hν S + O 4.8 10 9 2.24 1.1 10 8 2.01 2.1 10 9 1.47 → × − × − × − SO + hν SO+ + e 2.1 10 10 3.18 5.3 10 11 2.72 4.0 10 12 2.64 → − × − × − × − SO + hν SO + O 1.2 10 9 2.25 2.1 10 9 1.97 5.5 10 10 1.47 2 → × − × − × − SO + hν S + O 1.1 10 9 2.29 2.1 10 9 2.02 4.1 10 10 1.47 2 → 2 × − × − × − SO + hν SO+ + e 1.3 10 10 3.77 3.0 10 11 3.17 2.1 10 12 3.16 2 → 2 − × − × − × − HF + hν H + F 1.3 10 10 2.57 3.1 10 10 2.18 2.7 10 11 1.84 → × − × − × − HCl + hν H + Cl 2.0 10 9 2.53 7.7 10 10 1.99 4.2 10 10 1.68 → × − × − × − HCl + hν HCl+ + e 1.9 10 11 3.91 2.7 10 12 3.33 1.7 10 13 3.35 → − × − × − × − C + hν C+ + e 3.3 10 10 3.28 8.4 10 11 2.82 5.8 10 12 2.80 → − × − × − × − Si + hν Si+ + e 4.2 10 9 2.49 4.4 10 9 2.13 2.1 10 9 1.91 → − × − × − × − P + hν P+ + e 1.1 10 9 2.99 2.5 10 10 2.48 2.3 10 11 2.45 → − × − × − × − S + hν S+ + e 9.7 10 10 3.09 2.1 10 10 2.62 1.8 10 11 2.55 → − × − × − × − Cl + hν Cl+ + e 4.7 10 11 3.94 6.1 10 12 3.35 4.4 10 13 3.36 → − × − × − × − Na + hν Na+ + e 1.3 10 11 2.12 1.6 10 11 1.68 8.4 10 12 1.47 → − × − × − × − Mg + hν Mg+ + e 6.7 10 11 2.29 4.4 10 11 1.80 5.4 10 11 1.73 → − × − × − × − Fe + hν Fe+ + e 4.7 10 10 2.45 6.9 10 10 2.02 2.8 10 10 1.88 → − × − × − × − Notes. (a)Interstellar radiation field (ISRF) of Draine(1978); see expressions in Sect. 2.1.1. (b)Radiation field of TW Hya scaled to the FUV energy density of the ISRF. (c)Radiation field of AB Aurigae scaled to the FUV energy density of the ISRF.

A19, page 32 of 33 M. Agúndez et al.: The chemistry of disks around T Tauri and Herbig Ae/Be stars the Herbig Ae/Be radiation field some photoprocesses have rates Ae/Be radiation field, our parameters α and γ are also not very similar to those computed under the ISRF (within one order of different from those calculated by van Dishoeck et al.(2006) magnitude) while for others the rates are lower by more than and Heays et al.(2017) for a 10 000 K blackbody radiation a factor of ten. By looking to the spectral shape of the dif- field, because the FUV spectrum of AB Aurigae used by us ferent FUV radiation fields (see Fig. B.1) we note that, except is not drastically different from that of a 10 000 K blackbody. for the fact that stellar spectra have spectral features while the The same is not true in the case of the radiation field of the ISRF is a continuum, the spectra of the T Tauri star and the T Tauri star. The FUV spectrum of a 4000 K blackbody is very ISRF are more flat than that of the Herbig Ae/Be star, which different from that of TW Hya, which translates into very differ- shows a depletion of flux at short wavelengths (below 1300 Å). ent photodissociation and photoionization rates. In general, the Therefore, those photoprocesses which occur more effectively∼ at rates calculated by us for a T Tauri star are orders of magnitude short wavelengths, as, for example, the photodissociation of N2 higher than those computed by van Dishoeck et al.(2006) and (see Fig.A.1), have higher rates under the ISRF and the T Tauri Heays et al.(2017). radiation field than under the Herbig Ae/Be field. The values provided in Table B.1 can be useful in chem- It is interesting to compare our results with previous stud- ical models of protoplananetary disks around T Tauri and ies. In general, the unattenuated rates α and the attenuation Herbig Ae/Be stars, as long as they allow for computation of parameters γ calculated here under the ISRF are similar to those the photodissociation and photoionization rates in a simple way, calculated by Roberge et al.(1991), van Dishoeck et al.(2006), avoiding the more expensive approach, in terms of computing and Heays et al.(2017). As concerns the rates under the Herbig time, of solving the FUV radiative transfer.

A19, page 33 of 33