Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21725119p&link_type=abstract
American Astronomical Society, AAS Meeting #217, #251.19; Bulletin of the American Astronomical Society, Vol. 43, 2011
Astronomy and Astrophysics
Astronomy
Scientific paper
We investigate the formation and evolution of interstellar dust-grain ices under cold cloud conditions, with a particular emphasis on CO2. 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. We undertake to treat more accurately the quantum-tunneling rates of barrier-mediated surface reactions, and we explicitly consider competition between such reaction rates and thermal hopping processes. These models show excellent agreement with the observed behavior of CO and CO2 ice in the interstellar medium. The observed threshold between regimes in which CO2 or CO is the dominant ice constituent after H2O is found to be caused ultimately by the near-complete gas-phase conversion of atomic carbon to CO, which is itself determined by CO and H2 self-shielding. The change in the availability of gas-phase carbon alters the balance of the grain-surface chemistry, leading to a sharp change-over in the dominance of CO2/CO. The most probable grain-surface production mechanism for CO2 is the formation of a loosely-bound O...CO complex, whose oxygen atom is easily hydrogenated, leaving a highly excited complex which quickly overcomes an activation energy barrier to form CO2 + H.
Garrod Robin T.
Pauly Tyler
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