CO depletion: a microscopic perspective
  In regions where stars form, variations in density and temperature can cause
gas to freeze-out onto dust grains forming ice mantles, which influences the
chemical composition of a cloud. The aim of this paper is to understand in
detail the depletion (and desorption) of CO on (from) interstellar dust grains.
Experimental simulations were performed under two different (astrophysically
relevant) conditions. In parallel, Kinetic Monte Carlo simulations were used to
mimic the experimental conditions. In our experiments, CO molecules accrete
onto water ice at temperatures below 27 K, with a deposition rate that does not
depend on the substrate temperature. During the warm-up phase, the desorption
processes do exhibit subtle differences indicating the presence of weakly bound
CO molecules, therefore highlighting a low diffusion efficiency. IR
measurements following the ice thickness during the TPD confirm that diffusion
occurs at temperatures close to the desorption. Applied to astrophysical
conditions, in a pre-stellar core, the binding energies of CO molecules,
ranging between 300 K and 850 K, depend on the conditions at which CO has been
deposited. Because of this wide range of binding energies, the depletion of CO
as a function of AV is much less important than initially thought. The weakly
bound molecules, easily released into the gas phase through evaporation, change
the balance between accretion and desorption, which result in a larger
abundance of CO at high extinctions. In addition, weakly bound CO molecules are
also be more mobile, and this could increase the reactivity within interstellar
ices.
