Cyanide Photochemistry and Nitrogen Fractionation in the Mwc 480 Disk

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Citation Guzmán, V. V., K. I. Öberg, R. Loomis, and C. Qi. 2015. “Cyanide Photochemistry and Nitrogen Fractionation in the Mwc 480 Disk.” The Astrophysical Journal 814 (1) (November 17): 53. doi:10.1088/0004-637x/814/1/53.

Published Version doi:10.1088/0004-637x/814/1/53

Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:24820077

Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#OAP arXiv:1511.03313v1 [astro-ph.SR] 10 Nov 2015 esasaeodr fmgiuemr uiosta T than luminous more magnitude stars. of Herbig Tauri orders because unclear are is be stars stars should lack Ae Tauri structure a T similar around in a expected Whether resulting disk emission. absent, the HCN effectively region, of disk is the outer layer Ae in the midplane molecular In Herbig the ex- to disk. around down more low-density disks penetration outer much UV in to be emission due stars should HCN (2007) addition emission than al. In tended CN et Jonkheid 1993). that heights, ra- al. proposed UV scale et vertical of (Fuente different tracer disks to a in as fields proposed diation been has ratio below. CN/HCN introduced is regions as iso- disk molecules heavy cold 1997; specific of these into fractionation al. of efficient topes et to existence (Dutrey conductive atoms The be complete heavy may 2013). nearly of al. be et freeze-out cold Qi to the midplane expected and the surface is In disk the in readily present between midplane. mainly are layer are that protected radiation a Molecules UV by 2003). dissociated disk al. photo UV-illuminated CN) et the in (Bergin (e.g. survive surface radicals to and expected chemically Atoms are radiation a alone This structure. in disk result 2002). should stratified al. structure et temperature Herczeg gas and 1991; and al. dust by et radiation ra- (Calvet this interstellar of and attenuation stellar to and to diation, due surfaces disk gradients of radiation exposure the UV and temperature vertical tet abig,M 23,USA 02138, MA Cambridge, Street, [email protected] rpittpstuigL using 2015 typeset 12, Preprint November version Draft u oterdffrn eeso ht resistance, photo of levels different their to Due and radial by characterized are disks Protoplanetary 1 avr-mtsna etrfrAtohsc,6 Garden 60 Astrophysics, for Center Harvard-Smithsonian eel eydffrn ailaudnepolsfrC n C,wit HCN, and CN for emissio profiles HCN abundance clea of is radial > cut-off observed molecules different the all very to HC exterior from reveals of 1” Emission than detection Tau. more first radially DM the star including Tauri resolved, T around disk the nads.W n o ikaveraged disk low a find We disk. a in ujc headings: an Subject inheritance disk. interstellar this demonstrating in comets, and cores cloud stplgedt oadteMC40ds opoietefis meas first the provide to disk and are 480 Ae profiles MWC Herbig emission the in HCN toward and structures th data CN where photochemical isotopologue in disk different difference the of such of No suggestive outer-part low. the are in densities CN into photodissociation HCN HC h tlt fHNa rb fdss epeetAM observations ALMA present we disks, of of probe degree a its exposure as measuring UV by HCN of objects, of tracer of utility proposed history a the thermal is the CN to probe respect with abundance YND HTCEITYADNTOE RCINTO NTEM THE IN FRACTIONATION NITROGEN AND PHOTOCHEMISTRY CYANIDE 0 Uad110 and AU 300 C sacmol bevdmlcl nSlrSse oisadi inters in and bodies System Solar in molecule observed commonly a is HCN 15 br ta.(01 sd3 used (2011) al. et Oberg ¨ oadtepoolntr ikaon ebgA trMC8,a MWC480, star Ae Herbig around disk protoplanetary the toward N 1. INTRODUCTION A T E tl mltajv 05/12/14 v. emulateapj style X ± srceity S:mlcls rtpaeaydss ai lines radio disks, protoplanetary molecules, ISM: astrochemistry, nua resolution angular 0A,rsetvl.Ti euti nareetwt oe prediction model with agreement in is result This respectively. AU, 10 ..Guzm V.V. ′′ an ´ Submillimeter 1 rf eso oebr1,2015 12, November version Draft K.I. , 14 N / 15 ABSTRACT 15 Oberg ¨ ai f200 of ratio N nads.Twr W 8,C msinextends emission CN 480, MWC Toward disk. a in N 1 .Loomis R. , oad h rtpaeaydssaon 1 around disks protoplanetary the towards iktemlhsoy C sepce obcm en- become to expected is HCN in riched history. thermal disk lines. the CN of and characteristics HCN distribu- excitation radial observed different relative of the because constrain tion not could but star, evdC n C msina naglrrslto of ob- angular-resolution (2012) an al. at 1 emission et HCN Chapillon and cases. CN served of handful a in ferences trsdet ml ieec nzr-on energies zero-point in difference small the a for to due atures C,adteHNioooousH isotopologues HCN the and HCN, isotopic System Solar Nebula interpreting Solar to the data. Measurements key to therefore analogs Discussion). in are see fractionation and nitrogen of 2014, al. and et potential (Heays 2011, other Charnley however, of under- & are, sources to Mumma There used therein). (see been references origin have plan- their etc) gaseous stand planets, meteorites, rocky comets, in Sun, bodies ets, (the different System of Solar scenario, compositions the this on isotopic Based nitrogen 2008). the Charnley & Rodgers 2000; esa W 8.W lopeetC n C data HCN and CN 0 present around also disk We the toward 480. MWC star Ae fdsst erhfrdffrne nteC n HCN and CN the in 1 differences low-significance sample identified for and a search radii, in emission HCN to and disks CN of of observations (SMA) Array omn eina itneo 4 c n r nw to known are and star pc, Taurus 140 the of in distance a located at are region forming stars pho- Ae Both Herbig and Tauri tochemistry. T between comparison a enable ′′ C stplgerto r oeta rbso the of probes potential are ratios isotopologue HCN nti ae epeetAM bevtoso Nand CN of observations ALMA present we paper this In . 5 − ± 3 ′′ 14 0,cmaal owa sosre in observed is what to comparable 100, 15 oad w ar tr n n ebgAe Herbig one and stars Tauri T two towards and N 1 15 n .Qi C. and , ceial rcintd tlwtemper- low at fractionated) (chemically N /recetntoe fractionation nitrogen efficient d/or irgnfatoain oaddress To fractionation. nitrogen nihetta hmclfractionation chemical than enrichment N ar ik.W s h HCN the use We disks. Tauri T C sas rqetyue to used frequently also is HCN . .Qatttv oeigfurther modeling Quantitative n. etfi ue u-ffrdiof radii cut-off outer best-fit h etclgsadds column dust and gas vertical e 15 rmn fthe of urement stplge Triv Herbst & (Terzieva isotopologues N do NadHNtoward HCN and CN of nd fC,HN H HCN, CN, of bevdtwr MTau, DM toward observed l eetdadspatially and detected rly 1 elrevrnet.Its environments. tellar . 5 M ⊙ ehius high techniques: ; ar trD a to Tau DM star Tauri T C40DISK 480 WC 14 N fefficient of s / 13 13 15 Nand CN NadHC and CN ratio N . 8 M ⊙ − Herbig 2 σ 15 dif- N 2 V.V. Guzmán et al.

TABLE 1 Observation parameters.

Line Frequency Eu/k Beam PA Channel width Noise ◦ GHz K arcsec km s−1 mJy beam−1 channel−1 CN N = 3 − 2,J = 7/2 − 5/2 340.24777 32.6 0.78 × 0.45/0.65 × 0.48 −23.1/ − 29.2 0.1 12.3/9.2 HCN J = 4 − 3 354.50548 42.5 0.77 × 0.41/0.64 × 0.40 −24.1/ − 29.0 0.1 14.0/11.2 H13CN J = 3 − 2 259.0118 24.9 0.74 × 0.46 −8.8′′ 0.2 3.2 HC15N J = 3 − 2 258.1571 24.8 0.74 × 0.46 −9.0′′ 0.2 4.2

Note: Two values are given for the beam, PA and noise, corresponding to the parameters in MWC 480 and DM Tau, respectively.

CN(3 − 2) MWC 480 J = 7/2 − 5/2

2.00 2.75 3.50 4.25 5.00 5.75 6.50 7.25 8.00 2 HCN(4 − 3) 1 )

” 0 ( y δ −1 −2 2.00 2.75 3.50 4.25 5.00 5.75 6.50 7.25 8.00 12 0 −1 −2 δx(”) CN(3 − 2) DM Tau J = 7/2 − 5/2

4.40 4.80 5.20 5.60 6.00 6.40 6.80 7.20 7.60 2 HCN(4 − 3) 1 )

” 0 ( y δ −1 −2 4.40 4.80 5.20 5.60 6.00 6.40 6.80 7.20 7.60 12 0 −1 −2 δx(”) Fig. 1.— Channel maps of the HCN(4-3) and CN(3-2) line emission in MWC 480 (top) and DM Tau (bottom). The contour levels correspond to 3, 5, 7, 10, 15, 20 and 25σ (Table 1). Synthesized beams are shown in the bottom left corner of the first panels. present large, chemically rich disks (Dutrey et al. 1997; HCN lines. Henning et al. 2010; Oberg¨ et al. 2010; Chapillon et al. The data were calibrated by the ALMA staff us- 2012; Oberg¨ et al. 2015). The observations are data re- ing standard procedures, and retrieved from the public duction process are described in section 2. The obser- archive. We further self-calibrated the data in phase and vational results and analysis are presented in section 3 amplitude in CASA, taking advantage of the bright con- and a discussion is given in section 4. We summarize our tinuum emission in both sources. The solutions of the findings an conclude in section 5. self-calibration derived for the continuum were applied to each spectral window. The continuum was then sub- 2. OBSERVATIONS AND DATA REDUCTION tracted from the visibilities using channels free of line emission to produce the spectral line cubes. The images 2.1. CN and HCN were obtained by deconvolution of the visibilities using The HCN J =4 − 3 and CN N =3 − 2, J =7/2 − 5/2 the CLEAN algorithm in CASA, with Briggs weighting lines were observed towards MWC 480 and DM Tau with and a robust parameter of 0.5, resulting in an angular the Atacama Large (sub-)Millimeter Array (ALMA) dur- resolution of 0.6′′. A clean mask was created for each ing Cycle 0 (PI: E. Chapillon, proposal 2011.0.00629). channel to select only regions with line emission during The Band 7 observations were carried out in October the cleaning process. Table 1 summarizes the observa- 2012 with 23 antennas. The total on-source observing tional parameters of the lines. time was 34 min and the baselines lengths ranged between 21 and 384 m. The frequency bandpass was cali- 2.2. H13CN and HC15N brated by observing the bright quasar J0423-013 at the 13 − 15 − begining of the observing run. The phase and amplitude The H CN J =3 2 and HC N J =3 2 lines were temporal variations were calibrated by regularly observ- observed towards MWC 480 with ALMA during Cycle 2 ing the nearby quasars J0423-013, J01510-180 and Cal- as part of project 2013.1.00226 (PI: K.I. Oberg).¨ These listo. J01510-180 was used to derive the absolute flux observations, partially described in Oberg¨ et al. (2015), scale. Two spectral windows of 468 MHz bandwidth and are part of a larger survey that did not include DM Tau. 122 kHz spectral resolution were centered on the CN and In brief, the Band 6 observations were carried out in June Cyanide photochemistry and fractionation in the MWC 480 disk 3

3 3 MWC 480 DM Tau 2 2 1 1 ) ) ” ”

( 0 ( 0 y y δ −1 δ −1 −2 −2 −3 −3 3 12 0 −1 −2 −3 0.5 3 12 0 −1 −2 −3 0.08 δx(”) − − δx(”) − − 1.2 HCN(4 3) CN(3 2) 0.4 HCN(4 3) CN(3 2) J = 7/2 − 5/2 0.3 J = 7/2 − 5/2 0.06 ×2 0.8 0.3 0.2 Outer ring 0.04 0.2 158 AU 0.4 66 AU 102 AU 0.1 0.1 151 AU 0.02 Cont. flux(Jy) Cont. flux(Jy) Line flux(Jykm/s) Line flux(Jykm/s) 0.0 0.0 0.0 0.00 0 100 200 0 100 200 0 200 400 0 200 400 Radius (AU) Radius (AU) Fig. 2.— Dust continuum emission, moment zero maps (upper panels) and azimuthally-averaged emission profiles (bottom panels) of the HCN J = 4 − 3 and CN N = 3 − 2, J = 7/2 − 5/2 lines (blue solid lines) in MWC480 (left) and DM Tau (right). The azimuthally-averaged dust continuum profiles are shown in dashed lines. The weaker HCN line in DM Tau has been multiplied by 2 to show the radial profile, and the outer ring near 300 AU, more clearly. Synthesized beams are shown in the bottom left panel corners. The double-arrows mark the half-light radii.

2014 with 33 antennas with baseline lengths spanning be- TABLE 2 tween 18 and 650 m. The total on-source observing time Adopted parameters in disk model for MWC 480. was 24 min. The quasar J0510+1800 was observed to calibrate the amplitude and phase temporal variations, Parameter Value Ref. H100 16 AU as well as the frequency bandpass. The same quasar was Scale height Guilloteau et al. (2011) used to derive the absolute flux scale, as no planet was h 1.25 available during the observations. Two spectral windows Mgas 0.18M⊙ of 59 MHz bandwidth and 61 kHz channel spacing cov- Surface density γ 0.75 Guilloteau et al. (2011) ered the H13CN and HC15N lines. Rc 81 AU T100 23 K A description of the phase and amplitude calibra- Piétu et al. (2007) tion, as well as the self-calibration, can be found in Temperature q 0.5 β 1.5 Dartois et al. (2003) Oberg¨ et al. (2015). The self-calibrated visibilities were cleaned in CASA using a robust parameter of 1.0, yield- ′′ ing an angular resolution of ∼ 0.6 (see Table 1). This compared to HCN. The best-fit half-light radius for the selection of robustness parameter allowed us to improve HCN and CN emission toward MWC 480 are 66 and the signal-to-noise ratio of the weak H13CN and HC15N 102 AU, respectively. We further note that the HCN lines, while maintaining a good angular resolution. A emission has a similar size to that of the dust continuum clean mask was created for each channel, similarly to emission, which is shown in dashed-line in Fig. 2. In con- CN and HCN, to select only regions with line emission trast, toward DM Tau the best-fit half-radius for HCN during the cleaning process. and CN are similar, 151 and 158 AU, respectively. The 3. OBSERVATIONAL RESULTS AND ANALYSIS HCN outer ring hinted at in the DM Tau channel maps is also confirmed as a ‘bump’ around 300 AU in the radial 3.1. Spatial distribution of CN and HCN profile. No such ring is seen toward MWC 480. Figure 1 shows the channel maps of the CN N =3−2, To quantify the HCN and CN radial abundance profiles J =7/2 − 5/2 and HCN J =4 − 3 lines in the MWC 480 we modeled the CN and HCN emission using the pro- and DM Tau disks. The velocity pattern characteristic cedure and disk density and temperature structure de- of Keplerian rotation is seen for both lines and disks. To- scribed in (Oberg¨ et al. 2015). Key parameters adopted ward MWC 480, the HCN emission is clearly more com- in the model are listed in Table 2. In brief, we model pact than the CN emission in all channels. Such a differ- molecular abundances by defining them as power-laws α ence in emission patterns is not observed toward the DM X = X0(r/R0) , with X0 the abundance with respect Tau disk. Furthermore, DM Tau exhibits a much more to total hydrogen at R0 = 100 AU. We also included complex emission pattern in both the CN and HCN lines. an outer cut-off radius, Rout. We ran grids of mod- The CN ‘butterfly’ shape in some of the central channels els for different outer radius 50 < Rout/AU < 500, is caused by spectrally resolved hyperfine components. power-law indexes −2 <α< 2 and molecular abun- −14 −2 −9 HCN seems to present an outer ring, whose potential dances 10 < X100/ cm < 10 . The line emission origin is discussed in section 4. was calculated using the radiative transfer code LIME Fig. 2 shows the continuum, and velocity integrated (Brinch & Hogerheijde 2010) assuming non-LTE excita- maps and azimuthally averaged radial profiles of the CN tion with collision rates from (Dumouchel et al. 2010) for and HCN lines towards MWC 480 and DM Tau. To- HCN and from Lique et al. (2010) for CN. For the HCN ward MWC 480, these data representations confirm the isotopologues, we use the collisional rates of HCN. The noted differences in CN and HCN emission regions in model fitting was done in the u − v plane by computing Fig. 1. CN emission is detectable at a 2× larger radius χ2, the weighted difference between model and observa- 4 V.V. Guzmán et al.

HCN(4 − 3)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Model

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

2 Residual 1 )

” 0 ( y

δ −1 −2 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 12 0 −1−2 δx(”) CN(3 − 2)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Model

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

2 Residual 1 )

” 0 ( y

δ −1 −2 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 12 0 −1−2 δx(”) Fig. 3.— Channel maps of the observed CN and HCN emission in MWC 480 (upper panels), together with synthetic observations generated −10 −12 from the best-ﬁt models for each line, corresponding to X0 = 1.0 × 10 , Rout = 350 AU and α = −1 for CN, and X0 = 5.0 × 10 , Rout = 110 AU and α = −1 for HCN (middle panel). The residuals are shown in the bottom panels. The contour levels correspond to 3, 5, 7, 10, 15, 20 and 25σ.

500 500 rectly compared with model predictions. Table 3.2 lists CN HCN the best ﬁt parameters together with what are formally 400 400 7σ error bars based on the data SNR. In our case the data 300 300 SNR is high enough that we are dominated by stochastic

out out noise from the radiative transfer code and the 7σ er- R 200 R 200 ror bar represents the deviation from the best fit model 100 100 where there is a clear, visual misfit between model and data. 0 0 −2 −1 0 1 −2 −1 0 1 Figure 3 shows the excellent fit between our best-fit α α model and the data. Except for the low-velocity CN channels there are no significant residuals, and those Fig. 4.— Contour plots of the χ2 surface for CN (left) and HCN residuals are due to the second hyperfine component of (right) abundance profiles in the MWC 480 disk. The abundance CN, which is not included in our model. This hyperfine at 100 AU (X0) is fixed to the best-fit model value (see Table 3). component is weak in MWC 480, but it is clearly seen in The thick contour corresponds to the 7σ contour level, and the light-gray lines mark contours with ∆χ2 = 100, 200, 300 and 400. DM Tau (see Fig. 1). The good fit is also apparent when comparing the disk-integrated spectra (Fig. 5), where the spectra are extracted using a Keplerian mask. tions, for the real and imaginary parts of the complex visibilities. The best-fit models were obtained by mini- mizing χ2. 3.2. HCN isotopologues Fig. 4 shows the resulting contours of the ∆χ2 = Figure 6 displays the moment-zero maps of the H13CN 2 2 15 χ − χmin distribution for the HCN and CN abundance and HC N lines in MWC 480 together with the inte- profiles. The best fit model of both CN and HCN lines grated emission of the blue and red shifted part of the corresponds to a decreasing abundance profile as a func- the spectra, illustrating the disk rotation. The emission tion of radius, with α = −1. The best-fit outer-radius of of the two isotopologues is compact, visually similar to the HCN emission is 110 AU, while the outer-radius of the dust continuum and the HCN line emission. the CN emission is constrained to > 300 AU, i.e. > 2× We modeled the emission of the HCN isotopologues larger than HCN. The best-fit abundances at 100 AU of following the same procedure as for CN and HCN above. CN and HCN are given in Table 3 but they are highly Figure 8 shows the ∆χ2 distribution for the H13CN and model dependent, i.e. they depend strongly on the as- HC15N abundance profiles. The best fit parameters are sumed vertical abundance profile, and should not be di- listed in Table 3. The good fit of the best-fit model to Cyanide photochemistry and fractionation in the MWC 480 disk 5

1200 1200 30 30 HCN(4-3) CN(3-2) H13CN(3-2) HC15N(3-2)

800 800 20 20

10 10 400 400 Flux [mJy] Flux [mJy]

0 0 0 0 −10 0 10 20 −10 0 10 20 −10 0 10 20 −10 0 10 20 Velocity [km/s] Velocity [km/s] Velocity [km/s] Velocity [km/s]

Fig. 5.— HCN and CN spectra integrated over the MWC 480 Fig. 7.— H13CN and HC15N integrated spectra over the MWC disk (black histograms), with the best-fit models overlaid on top 480 disk (black histograms) with the best-fit models overlaid on (filled histograms). top (filled histograms).

160 160 13 − 15 − 13 15 H CN(3 2) HC N(3 2) 140 H CN140 HC N

120 120 out out

R 100 R 100

80 80

60 60 −2 −1 0 1 −2 −1 0 1 α α 1 Fig. 8.— Contour plots of the χ2 surface for H13CN (left) and HC15N (right) abundance proﬁles in the MWC 480 disk. The )

” 0 abundance at 100 AU (X0) is fixed to the best-fit model value ( y (see Table 3). The three contours correspond to 1σ, 2σ and 3σ δ confidence levels. −1 and density (Roueff et al. 2015). The main uncertainty (∼ 85%), however, arises from the error in the measured 1 0 −1 H13CN/HC15N ratio. δx(”) 4. DISCUSSION 13 − Fig. 6.— Moment zero maps (left panels) of the H CN J = 3 2 4.1. Photochemistry: CN and HCN and HC15N J = 3 − 2 lines in the MWC 480 disk, shown together with integrated flux maps in two velocity bins around the vlsr The observed CN and HCN disk emission profiles in (right panels), showing the velocity pattern. the MWC 480 disk are well-fit by power-law abundance profiles with a radial cut-off that is two times larger for the data disk integrated spectra (extracted using a Ke- CN compared to HCN. CN is thus significantly more ra- plerian mask) is shown in Fig. 7. The best-fit model dially extended than HCN, i.e. we can exclude with a abundances at 100 AU are given in Table 3. Similarly high level of confidence that excitation alone is respon- to CN and HCN, the absolute abundances are uncertain, sible for the different HCN and CN emission profiles. due to an unknown vertical abundance profile. Assum- This result is consistent with chemical model predic- ing that the HCN isotopologue emission originates in the tions of survival of CN and rapid photodissociation of same disk layer and that the lines are optically thin, their HCN in the outer, low-density disk (Aikawa et al. 2002; abundance ratio should be robust, however. This abun- Jonkheid et al. 2007). Indeed, these models predict that dance ratio is 2.8±1.4 and holds for the entire disk out to the CN abundance can remain constant and even increase 100 AU since both lines are best fit by the same power- with radius because radical formation is favored at low law index of −0.5. densities, and because CN is a major HCN photodisso- In the MWC 480 disk, the integrated H13CN/HCN ciation product. HCN, on the other hand, should only density flux ratio is ∼ 36, which is lower than the be present in the high-density parts of the disk where a 12C/13C isotopic ratios observed in the ISM, indicating shielded molecular layer exists. that the HCN line is partially optically thick. We there- The same models also predict a different vertical struc- fore use H13CN as a proxy of HCN to calculate the ni- ture of CN and HCN, with the CN abundance peak- trogen fractionation in HCN. Assuming an isotopic ratio ing close to the disk surface in the inner disk regions. of 12C/13C= 70, we derive a disk HCN isotopologue ra- Constraining the vertical abundances of these molecules tio HC14N/HC15N = 200 ± 110. To estimate this error requires multiple line transitions and/or direct observa- we included a 30% uncertainty in the C isotopic ratio, tions of vertical emission patterns in edge-on disks. In as this value has been shown to vary both with time the meantime, the derived abundance ratio of the two 6 V.V. Guzmán et al.

TABLE 3 Best-fit parameters for the modeled abundance profiles in MWC 480.

CN HCN H13CN HC15N −11 −12 −13 −14 X0 (8.5 ± 1.5) × 10 (5.2 ± 0.7) × 10 (1.3 ± 0.5) × 10 (4.7 ± 1.7) × 10 α -1 -1 -0.5 -0.5 Rout [AU] > 300 110 ± 10 100 ± 15 95 ± 30

Note: The absolute abundances are largely uncertain, due to the unknown vertical abundance and temperature profiles. However, because H13CN and HC15N are expected to be, for the most part, co-spatial their abundance ratio does not suffer greatly from uncertainties in the vertical temperature profile. Therefore, the inferred H13CN/HC15N ratios is robust.

600 ratio in our Solar System reveal a clear diﬀerence between gaseous and rocky bodies (Mumma & Charnley 500 2011). A low N fractionation (i.e., high 14N/15N) 400 Cold gas is observed toward the Sun and Jupiter (Marty et al. 14 15 N Disk 2011), while a high fractionation (i.e., low N/ N)

15 300 Comets / is observed in the rocky planets, comets and mete- N orites (Bockelée-Morvan et al. 2008). Isotopic ratios in 14 200 the coma of comets are determined remotely through 100 high-resolution radio and optical spectroscopy. The 0 ROSETTA space mission will hopefully provide in-situ measurements of the 14N/15N ratio in the coma of L183 Earth L1544 comet 67P/Churyumov-Gerasimenko. Comets display Holmes L1521E Diffuse 15 Solar wind Hale-Bopp MWC480 the highest N enhancements among Solar System bodies (Mumma & Charnley 2011). Prior to this study the 14 15 only other place where such high nitrogen fractionation Fig. 9.— N/ N isotopic ratios observed towards different So- lar system bodies (Marty et al. 2011; Bockelée-Morvan et al. 2008) is found is cold pre-stellar cores (Hily-Blant et al. 2013; and in the ISM (Hily-Blant et al. 2013; Lucas & Liszt 1998), and Wampfler et al. 2014). The cometary 15N abundance the disk around MWC 480. could thus signify an interstellar inheritance. Interstellar inheritance is not the only possibility, how- molecules remains relatively unconstrained. ever. Distinguishing between pre-Solar and Solar Nebula The emission of CN and HCN in the disk around T origins of nitrogen fractionation requires observations of tauri star DM Tau exhibit different patterns compared nitrogen fractionation in protoplanetary disks. In gen- to MWC 480. First, the spatial distribution (the ex- eral, HCN (isotopologue) formation begins with atomic tension of the emission) is similar for CN and HCN in nitrogen. At low temperatures 15N is preferentially in- the DM Tau disk. This difference between MWC 480 corporated into cyanides resulting in fractionation. Ob- and DM Tau may be explained by the fact that the UV servations of HCN and HNC fractionation toward pro- field around DM Tau should be substantially lower. Less tostars present a tentative trend of the 14N/15N with vertical disk column may therefore be required to shield temperature, supporting this scenario (Wampfler et al. the outer disk regions, resulting in a shielded molecular 2014). The second mechanism to produce an enhance- layer out to many 100s of AU in the DM Tau disk. Sec- ment in 15N in HCN is isotope selective photodissociation ond, HCN exhibits a low SNR outer ring around 300 AU, 14 15 14 of N N over N2, resulting in an enhanced atomic best identified in the azimuthally-averaged emission pro- 15N/14N ratio. Theoretical models have shown that this file. The gap demarcating the main HCN disk from mechanism can increase the 14N/15N ratio by up to a the outer ring is faintly seen in CN as well. This gap factor 10 in protoplanetary disks (Heays et al. 2014). may be the result of micron-sized dust depletion at the Pre-stellar inheritance, low-temperature chemical frac- same radius due to dust dynamics, which would enable tionation in cold disk regions, and photochemically UV radiation to penetrate down toward the midplane, driven fractionation in disk atmospheres could thus all photo-dissociating HCN. Such a dust lane has been ob- explain the observed nitrogen fractionation in comets. served toward TW Hya in scattered light (Debes et al. The presented HCN isotopologue observations in the 2013). Similar high-quality scattered light observations MWC 480 disk cannot distinguish between these sce- toward DM Tau could resolve this question. If this scenarios. Future, higher SNR and/or higher resolution nario is confirmed, these ALMA observations suggest disk studies could, however. The three scenarios should that CN/HCN observations could be developed into a present very different radial and vertical 14N/15N pro- powerful probe of dust distributions and dynamics in the files. Inheritance should produce an almost uniform ra- outer regions of disks. tio across the disk. Low-temperature chemical fractionation should result in a lower 14N/15N ratio in HCN in 4.2. Nitrogen fractionation in the MWC480 disk the outer, cold disk that is still protected from radiation We used observations of HCN isotopologues to deter- and also a decreasing ratio toward the disk midplane. mine the level of nitrogen fractionation in the MWC Finally, nitrogen fractionation from selective photodisso- 480 disk. Fig. 9 shows the measured 14N/15N ratio in ciation should result in the lowest 14N/15N ratio in HCN the MWC 480 disk compared with different Solar sys- closer to the disk atmosphere. In addition to detailed tem bodies and the ISM. Observations of the 14N/15N Cyanide photochemistry and fractionation in the MWC 480 disk 7 studies of individual sources, observations of the aver- which show that HCN is only abundant in disk regions aged 14N/15N ratio in disks exposed to different stellar where the radiation field has been attenuated sufficiently radiation fields will also bring insight into the origin of to prevent its photodissociation, while CN can survive in N fractionation. If low-temperature chemical fractina- the outer low-density, unshielded disk regions. This dif- tion is the dominant mechanism producing HC15N, then ference in CN and HCN radial extent is not seen toward a multi-source study should result in a trend of increas- the disk around T Tauri star DM Tau, suggestive of dif- ing 14N/15N ratio with stellar radiation field. In con- ferent photochemical structures in disks around low and trast, the opposite behaviour should be observed if most intermediate mass stars. The DM Tau disk also presents of the HC15N is produced by selective photodissociation an intriguing outer ring in HCN (and potentially in CN). in the surface layers. These observations will reveal how Observation of CN and HCN coupled to detailed model- frequently protoplanetary disks are seeded with 15N en- ing clearly have great potential as tracers of UV radiation riched molecules, and whether additional enrichment oc- field strengths across protoplanetary disks. curs in the disk phase, changing the fractionation pat- We also presented the first detection of the H13CN and tern during planet formation. The clear HC15N detec- HC15N isotopologues in a protoplanetary disk. We mod- tion in MWC 480 after 20 min integrated demonstrates eled the H13CN and HC15N emission, and derive a disk that such observations will be readily achievable toward average for the 14N/15N isotopic ratio of 200 ± 100 in a sample of disks in a reasonable time frame. MWC 480. This value, which is lower than the Solar one, is similar to what has been measured in comets and in the 5. SUMMARY AND CONCLUSIONS cold ISM. These observations show that with the current Spatially and spectrally resolved ALMA observations ALMA capabilities it is remarkable easy to detect the toward the protoplanetary disk surrounding Herbig Ae faint lines of rare istopologues of common molecules in star MWC 480 clearly show that its CN emission is ∼ 2 protoplanetary disks making new detections in the near times more extended than its HCN emission. We con- future very likely. Observations at higher sensitivity in MWC 480, as well as in other sources, should allow us to structed a parametric model of the disk to obtain the un- 14 15 derlying abundance profile of the two species and found resolve variations of the N/ N ratio across the disk, that the observed difference in emission profiles corre- and directly address the current fractionation puzzle of spond to similarly different abundance profiles for CN why different objects in our Solar system present differ- and HCN. This confirms theoretical model predictions ent N enrichment levels.

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