ON THE FORMATION OF CO{sub 2} AND OTHER INTERSTELLAR ICES

Journal Article · Fri Jul 01 00:00:00 EDT 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/735/1/15· OSTI ID:21576555

Garrod, R T ^[1]; Pauly, T ^[2]

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1. Department of Astronomy, Cornell University, Ithaca, NY 14853 (United States)
2. Department of Physics and Astronomy, Iowa State University, Ames, IA 50011 (United States)

We investigate the formation and evolution of interstellar dust-grain ices under dark-cloud conditions, with a particular emphasis on CO{sub 2}. We use a three-phase model (gas/surface/mantle) to simulate the coupled gas-grain chemistry, allowing the distinction of the chemically active surface from the ice layers preserved in the mantle beneath. The model includes a treatment of the competition between barrier-mediated surface reactions and thermal-hopping processes. The results show excellent agreement with the observed behavior of CO{sub 2}, CO, and water ice in the interstellar medium. The reaction of the OH radical with CO is found to be efficient enough to account for CO{sub 2} ice production in dark clouds. At low visual extinctions, with dust temperatures {approx}>12 K, CO{sub 2} is formed by direct diffusion and reaction of CO with OH; we associate the resultant CO{sub 2}-rich ice with the observational polar CO{sub 2} signature. CH{sub 4} ice is well correlated with this component. At higher extinctions, with lower dust temperatures, CO is relatively immobile and thus abundant; however, the reaction of H and O atop a CO molecule allows OH and CO to meet rapidly enough to produce a CO:CO{sub 2} ratio in the range {approx}2-4, which we associate with apolar signatures. We suggest that the observational apolar CO{sub 2}/CO ice signatures in dark clouds result from a strongly segregated CO:H{sub 2}O ice, in which CO{sub 2} resides almost exclusively within the CO component. Observed visual-extinction thresholds for CO{sub 2}, CO, and H{sub 2}O are well reproduced by depth-dependent models. Methanol formation is found to be strongly sensitive to dynamical timescales and dust temperatures.

• OSTI ID: 21576555
• Journal Information: Astrophysical Journal, Vol. 735, Issue 1; Other Information: DOI: 10.1088/0004-637X/735/1/15; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
CARBON DIOXIDE
CARBON MONOXIDE
CHEMICAL REACTIONS
CLOUDS
DIFFUSION
DUSTS
HYDROXYL RADICALS
ICE
INTERSTELLAR SPACE
METHANE
METHANOL
WATER
ALCOHOLS
ALKANES
CARBON COMPOUNDS
CARBON OXIDES
CHALCOGENIDES
HYDROCARBONS
HYDROGEN COMPOUNDS
HYDROXY COMPOUNDS
ORGANIC COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
RADICALS
SPACE