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Photodissociation and Photoionisation of Atoms and Molecules of Astrophysical Interest A A&A 602, A105 (2017) Astronomy DOI: 10.1051/0004-6361/201628742 & c ESO 2017 Astrophysics Photodissociation and photoionisation of atoms and molecules of astrophysical interest A. N. Heays?, A. D. Bosman, and E. F. van Dishoeck Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: [email protected] Received 19 April 2016 / Accepted 1 January 2017 ABSTRACT A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic physical processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and for other wavelength-dependent radiation fields, including cool stellar and solar radiation, Lyman-α dominated radiation, and a cosmic-ray induced ultraviolet flux. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. Comparison with other databases such as PHIDRATES is made. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic C, and self-shielding column densities. The relative importance of these shielding types depends on the atom or molecule in question and the assumed dust optical properties. All of the new data are publicly available from the Leiden photodissociation and ionisation database. Sensitivity of the calculated rates to variation of temperature and isotope, and uncertainties in measured or calculated cross sections, are tested and discussed. Tests were conducted on the new rates with an interstellar-cloud chemical model, and find general agreement (within a factor of two) in abundances obtained with the previous iteration of the Leiden database assuming an ISRF, and order-of-magnitude variations assuming various kinds of stellar radiation. The newly parameterised dust-shielding factors makes a factor-of-two difference to many atomic and molecular abundances relative to parameters currently in the UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray induced photodissociation and ionisation rates differ from current standard values up to a factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates alter the equilibrium abundances of abundant dark cloud abundances by up to a factor of two. The partial cross sections for H2O and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead to radiation-field-dependent branching ratios. Key words. photon-dominated region (PDR) – cosmic rays – dust, extinction – ISM: molecules – molecular data – atomic data 1. Introduction The abundant UV photons in these regions photodissoci- ate and photoionise the main hydrogen, carbon, oxygen and Ultraviolet (UV) photons play a critical role in interstellar and nitrogen-containing species, controlling the H+ ! H ! circumstellar chemistry. The realisation that photodissociation + H2,C ! C ! CO, O ! O2 and N ! N2 transi- and photoionisation processes control the abundances of atoms tions (Tielens & Hollenbach 1985; van Dishoeck & Black 1988; and small molecules in diffuse interstellar clouds dates back Li et al. 2013). Photoprocesses thus affect the abundance of the nearly a century (Eddington 1928; Kramers & Ter Haar 1946; main cooling species in the interstellar medium, and they also Bates & Spitzer 1951). Similarly, photodissociation of parent generate chemically-reactive ions and radicals, opening path- species by UV radiation from the Sun has long been known ways to the formation of larger species (Sternberg & Dalgarno to explain the existence of small molecules in cometary comae 1995; Lee et al. 1996; Jansen et al. 1996; Li et al. 2014). The (Haser 1957; Crovisier et al. 1997). Nowadays, photodissocia- gas-phase abundance of more complex molecules formed tion processes are found to be important for modelling the chem- in this way is simultaneously limited by their own pho- istry of nearly every type of astrophysical region, from the edges todestruction (Teyssier et al. 2004; van Hemert & van Dishoeck of dense clouds close to bright young stars to the surface layers 2008; Guzman et al. 2014). The photoionisation of atoms and of protoplanetary disks, envelopes around evolved stars and gi- molecules also leads to a significant speed up of PDR chemistry ant molecular clouds on galactic scales (e.g., Glassgold 1996; due to the enhanced reaction rates of ions compared with neutral Hollenbach & Tielens 1999; Tielens 2013; van Dishoeck et al. species (Tielens 2013; van Dishoeck 2014). 2006; Glover & Clark 2012). Such clouds of gas and dust in which photodissociation is the dominant molecular destruction The quantitative modelling of chemical evolution in clouds, path are termed photodissociation or photon-dominated regions envelopes and disks is a prerequisite for the full interpreta- (PDRs), although the term PDRs originally referred mostly to tion of observations of their emitting molecular lines and dust high density regions close to bright O and B stars such as found continuum. Such models consider many physical regimes (e.g., in Orion (Tielens & Hollenbach 1985). Le Petit et al. 2006; Walsh et al. 2013) and involve many classes of chemical reactions (Wakelam et al. 2012; McElroy et al. ? Current contact: Observatoire de Paris, LERMA, UMR 8112 du 2013). By quantitatively constraining the rates of photopro- CNRS, 92195 Meudon, France. cesses, as is done in this paper, other chemical and physical Article published by EDP Sciences A105, page 1 of 62 A&A 602, A105 (2017) parameters processes affecting observations can be more reliably extended by new additions of complex-organic species that have determined. recently been detected in the interstellar medium3. The fundamental quantities governing photodissociation and ionisation are the wavelength-dependent flux of incident UV ra- 2. Radiation fields diation, discussed in Sect.2, and the wavelength-dependent pho- The photodissociation or photoionisation rate toabsorption, photodissociation, and photoionisation cross sec- −1 −1 tions of each atom or molecule, introduced in Sects.3 and4. (molec. atom s ) of a molecule (or atom) exposed to an Historically, the complete and unabridged specification of these ultraviolet radiation field is quantities contained too much information to be included in as- Z trochemical models, and is actually in many cases unnecessary k = σ(λ)I(λ)dλ, (1) given the scale of uncertainties in observations and other model parameters. Tabulated pre-integrations of the full wavelength de- where σ(λ) is the appropriate photodissociation or photoioni- pendence into a process rate (or lifetime) for different species in sation cross section, to be discussed in Sect.3, and I(λ) is the different kinds of UV-irradiated environments are useful to speed photon-based radiation intensity summed over all incidence an- up modelling. We calculated such rates in Sect.5. Such tabula- gles. A photon-counting intensity was used for calculations in tions must necessarily include the column-density-dependent ef- this paper because of the discrete nature of photodestruction fect of radiation shielding by dust, H and H2 inside interstellar events, but is directly related to the volumetric radiation energy and circumstellar clouds. The wavelength dependence of such density according to U(λ) = hI(λ)/λ where h is the Planck’s con- shielding is frequently presented by a simplified parameterisa- stant. An angularly-differential radiation intensity may be appro- tion and is discussed further in Sect.6. priate if the incident radiation is non-isotropic. The integration Astrochemical models can also use the full molecular and limits in Eq. (1) are defined by the wavelength range correspond- atomic cross sections as functions of wavelength, and con- ing to the nonzero photodissociation or ionisation cross section sider the dissociation of species and shielding by H and H2 and radiation intensity. line-by-line, to compute the photodestruction of molecules as The average intensity of the interstellar radiation field (ISRF) functions of depth into a PDR (e.g., van Dishoeck & Black can be estimated from the number and distribution of hot stars 1988; Viala et al. 1988; Jansen et al. 1995; Le Petit et al. 2006; in the Galaxy, combined with a model for the dust distribution Woitke et al. 2009; Walsh et al. 2013; Li et al. 2013). Further- and its extinction of the stellar radiation (Habing 1968; Draine more, astrochemical programs that employ simplified rates for 1978; Mathis et al. 1983; Parravano et al. 2003). The various es- photodestruction may require precomputing many of these when timates of the mean UV energy density at a typical point in the exploring, for example, a range of possible dust grain ultraviolet local galaxy agree to within a factor of two. Variations in this extinction properties (e.g., van Dishoeck et al. 2006; Röllig et al. energy density of a factor between two and three are expected 2013). Fundamental atomic and molecular cross sections such as throughout the galactic plane and on time scales of a few Gyr, those presented here are then required. as massive O and B star clusters form and die. In addition, the Even deep inside dark clouds well shielded from external ra- intensity ratio of short-wavelength photons capable of dissociat- diation, a weak UV flux is maintained. This is induced by the ing H2, CO and N2 and ionising atomic C (λ < 110 nm) and the interaction of cosmic rays with hydrogen. The resulting spec- broader far-ultraviolet range (91:2 < λ < 200 nm) may vary by a trum is highly structured (Prasad & Tarafdar 1983; Gredel et al. factor of two in location and time (Parravano et al. 2003). 1987) and incorporation of this process into astrochemical mod- The wavelength dependent UV intensity as defined by els also benefits from a reduction of the full wavelength depen- Draine(1978) is often adopted in astrochemical models, and dence into a conveniently tabulated rate. The most-recent tabu- given by the formula lation of these rates is by Gredel et al.(1989).
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