The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks

By R. Visser, E. Dishoeck, J. K. L. Observatory, Mpe Garching, O. Observatory. 2009

430 citations recorded by the source corpus.

Aims. Photodissociation by UV light is an important destruction mechanism for carbon monoxide (CO) in many astrophysical environments, ranging from interstellar clouds to protoplanetary disks. The aim of this work is to gain a better understanding of the depth dependence and isotope-selective nature of this process. Methods. We present a photodissociation model based on recent spectroscopic data from the literature, which allows us to compute depth-dependent and isotope-selective photodissociation rates at higher accuracy than in previous work. The model includes self-shielding, mutual shielding and shielding by atomic and molecular hydrogen, and it is the first such model to include the rare isotopologues C17O and 13C17O. We couple it to a simple chemical network to analyse CO abundances in diffuse and translucent clouds, photon-dominated regions, and circumstellar disks. Results. The photodissociation rate in the unattenuated interstellar radiation field is 2.6 $\times$ 10-10 s-1, 30% higher than currently adopted values. Increasing the excitation temperature or the Doppler width can reduce the photodissociation rates and the isotopic selectivity by as much as a factor of three for temperatures above 100 K. The model reproduces column densities observed towards diffuse clouds and PDRs, and it offers an explanation for both the enhanced and the reduced N(12CO)/N(13CO) ratios seen in diffuse clouds. The photodissociation of C17O and 13C17O shows almost exactly the same depth dependence as that of C18O and 13C18O, respectively, so 17O and 18O are equally fractionated with respect to 16O. This supports the recent hypothesis that CO photodissociation in the solar nebula is responsible for the anomalous 17O and 18O abundances in meteorites. Grain growth in circumstellar disks can enhance the N(12CO)/N(C17O) and N(12CO)/N(C18O) ratios by a factor of ten relative to the initial isotopic abundances.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/b75b57aec05a9f428a45483b6309795159d3361f

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Citations in the corpus

A KINETIC DATABASE FOR ASTROCHEMISTRY (KIDA) (2012, background): “Note that new shielding functions have been published by Visser et al. (2009) and will be included in Nahoon in the future.”
Survival of molecular gas in cavities of transition disks - I. CO (2013, background): “For H2 or CO selfshielding to become active, a column of order 1015 cm−2 is sufficient (Draine & Bertoldi 1996, Visser et al. 2009).”
WEAK TURBULENCE IN THE HD 163296 PROTOPLANETARY DISK REVEALED BY ALMA CO OBSERVATIONS (2015, background): “The exact location of the photo-dissociation boundary may vary between different isotopologues due to selective self-shielding (Visser et al. 2009; Miotello et al. 2014).”
The warm gas atmosphere of the HD 100546 disk seen by Herschel - Evidence of a gas-rich, carbon-poor atmosphere? (2012, methodology): “Freeze-out, thermal, and non-thermal desorption Freeze-out and thermal or non-thermal desorption are implemented following Visser et al. (2009b) and references therein.”
Photodissociation and photoionisation of atoms and molecules of astrophysical interest (2017, background): “The most recent treatments of these three molecules are given in Sternberg et al. (2014), Visser et al. (2009), and Li et al. (2013); Heays et al. (2014b); respectively.”
Physical Processes in the Interstellar Medium (2014, methodology): “Adapted from Ferriére (2001), Caselli et al. (1998), Wolfire et al.”
A Kinematical Detection of Two Embedded Jupiter-mass Planets in HD 163296 (2018, neutral): “Above the 19 K isotherm and below a shielding column of 1.2 × 1021 H2 cm−2 (Visser et al. 2009) a molecular abundance of 8.67 × 10−8 was assumed for C18O.”
A SPITZER SURVEY OF MID-INFRARED MOLECULAR EMISSION FROM PROTOPLANETARY DISKS. I. DETECTION RATES (2010, background): “2009, ApJ, 703, 1964 Glassgold, A. E., Najita, J., & Igea, J. 2004, ApJ, 615, 972 Gorti, U., & Hollenbach, D. 2008, ApJ, 683, 287 Grady, C. A. et al. 1996, A&AS, 120, 157 Grankin, K. N., Melnikov, S. Y., Bouvier, J., Herbst, W., & Shevchenko, V. S. 2007, A&A, 461, 183 Gray, R. O., & Corbally, C. J. ”
A PARAMETRIC MODELING APPROACH TO MEASURING THE GAS MASSES OF CIRCUMSTELLAR DISKS (2013, background): “Visser et al. (2009) examined the complexities of this process, including the effects of dust, H, H2, CO mutual shielding, excitation, doppler broadening, and ionmolecule isotope exchange reactions.”
THE RELATION BETWEEN GAS AND DUST IN THE TAURUS MOLECULAR CLOUD (2010, methodology): “These effects can reduce [CO]/[H2] by up to three orders of magnitude (e.g. van Dishoeck & Black 1988; Liszt 2007; Visser et al. 2009).”
Molecular Clouds in the Milky Way (2015, background): “…is also required to attenuate the local radiation field in order to build up a significant abundance of CO. Dust provides the primary opacity element, whereas line opacities from 12CO, 13CO, and H2 add to the effectiveness of this shielding layer (van Dishoeck & Black 1988, Visser et al. 2009).”
A Herschel [C ii] Galactic plane survey - I. The global distribution of ISM gas components (2013, background): “The [CO]/[H2] abundance ratio is expected to show strong differences between UV-exposed and shielded regions as a result of the competition between the formation and destruction of CO (e.g. van Dishoeck & Black 1988; Visser et al. 2009).”
Resolved gas cavities in transitional disks inferred from CO isotopologs with ALMA (2015, background): “As CO isotopologs have lower abundances, they are not self-shielded until deeper into a cloud or disk (Bally & Langer 1982; van Dishoeck & Black 1988; Visser et al. 2009).”
THE PdBI ARCSECOND WHIRLPOOL SURVEY (PAWS). I. A CLOUD-SCALE/MULTI-WAVELENGTH VIEW OF THE INTERSTELLAR MEDIUM IN A GRAND-DESIGN SPIRAL GALAXY (2013, methodology): “Recently, Visser et al. (2009) up-dated the calculations from van Dishoeck & Black (1988) using the latest laboratory data and fully treating the heavier CO isotopologues 13CO,C18O, and13C17O.”
THE DARK MOLECULAR GAS (2010, background): “…2006 private communication7) is the rate coefficient for OH reacting with C+ (first reaction), xC+n is the C+ gas phase number density, I ′CO = 2.6 × 10−10 s−1 (Visser et al. 2009) is the unshielded photodissociation rate of CO in the Draine field, fCO is the shielding factor caused by CO 7see…”
The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime (2015, methodology): “In that case, the importance of molecular (or self) shielding increases relative to dust shielding (Visser et al. 2009; Li et al. 2013a).”