On the Formation of CO2 and Other Interstellar Ices
  We investigate the formation and evolution of interstellar dust-grain ices
under dark-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. 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
CO2, 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 CO2 ice
production in dark clouds. At low visual extinctions, with dust temperatures
~12 K, CO2 is formed by direct diffusion and reaction of CO with OH; we
associate the resultant CO2-rich ice with the observational polar CO2
signature. CH4 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:CO2 ratio in the range ~2--4, which we
associate with apolar signatures. We suggest that the observational apolar
CO2/CO ice signatures in dark clouds result from a strongly segregated CO:H2O
ice, in which CO2 resides almost exclusively within the CO component. Observed
visual-extinction thresholds for CO2, CO and H2O are well reproduced by
depth-dependent models. Methanol formation is found to be strongly sensitive to
dynamical timescales and dust temperatures.
