Chemically driven desorption of CO from icy grains in dark clouds

Abstract

The chemical desorption of an adsorbed CO molecule in the vicinity of H2-forming sites on cosmic dust grains in cold dense clouds is investigated theoretically, mainly using a model based on a classical molecular dynamics computational simulation. As a model surface for icy mantles of dust grains, an amorphous water ice slab is generated at 10 K, and the first and the second H atoms are thrown on to the model surface to reproduce the recombination process of the two H atoms, H + H → H2. Then, the time and space dependence of the local temperature increase of icy mantles caused by the release of H2 formation energy in the vicinity of H2-forming sites is examined. It is found that icy mantles are heated locally up to about 30 K in the surface region at R ≤ 4 Å and about 20 K at 4 ≤ R ≤ 6 Å, where R is the distance from the H2-forming site. The critical temperature of CO desorption is estimated to be about 20-30 K under conditions in typical dense clouds, which might be seen to be comparable to the above result. However, the lifetime of local heating of icy mantles is found to be too short, compared with the time-scale of CO desorption (1013 s) and that for H2 forming in the vicinity of an adsorbed CO molecule (more than 2 × 1013 s). Thus, it is found that the efficiency of chemical desorption of CO on a large dust grain is negligible. On the other hand, chemical desorption can occur on a small dust grain with size less than 20 Å.