Desorption from interstellar ices

The desorption of molecular species from ice mantles back into the gas phase in molecular clouds results from a variety of very poorly understood processes. We have investigated three mechanisms: desorption resulting from H₂ formation on grains, direct cosmic ray heating and cosmic ray-induced photodesorption. Whilst qualitative differences exist between these processes (essentially deriving from the assumptions concerning the species selectivity of the desorption and the assumed threshold adsorption energies, E _t), all the three processes are found to be potentially very significant in dark cloud conditions. It is therefore important that all three mechanisms should be considered in studies of molecular clouds in which freeze-out and desorption are believed to be important.
Employing a chemical model of a typical static molecular core and using likely estimates for the quantum yields of the three processes, we find that desorption by H₂ formation probably dominates over the other two mechanisms. However, the physics of the desorption processes and the nature of the dust grains and ice mantles are very poorly constrained. We therefore conclude that the best approach is to set empirical constraints on the desorption, based on observed molecular depletions – rather than try to establish the desorption efficiencies from purely theoretical considerations. Applying this method to one such object (L16 89B) yields upper limits to the desorption efficiencies that are consistent with our understanding of these mechanisms.