A Revised Description of the Cosmic Ray Induced Desorption of Interstellar Ices

Non-thermal desorption of ices on interstellar grains is required to explain observations of molecules that are not synthesized efficiently in the gas phase in cold dense clouds. Perhaps the most important non-thermal desorption mechanism is one induced by cosmic rays (CRs), which, when passing through a grain, heat it transiently to a high temperature - the grain cools back to its original equilibrium temperature via the (partial) evaporation of the ice. Current cosmic-ray-induced desorption (CRD) models assume that the ice consists of a homogeneous layer of a generic CO-like volatile molecule, leading to a fixed grain cooling time. In this work we present a revised description of CRD in which the desorption efficiency depends dynamically on the ice content. We apply the revised desorption scheme to two-phase and three-phase chemical models in physical conditions corresponding to starless and prestellar cores, and to molecular clouds surrounding the cores. We find that inside starless and prestellar cores, introducing dynamic CRD in general decreases gas-phase abundances in two-phase chemical models, and increases gas-phase abundances in three-phase chemical models - dynamic CRD helps to retain appreciable gas-phase abundances in three-phase chemical models. In molecular cloud conditions, we find variations in ice abundances that can exceed five orders of magnitude; dynamic CRD suppresses the formation of lightly-bound molecules in the ice at low visual extinctions. Further improved CRD models need to take into account additional effects in the transient heating of the grains, introduced for example by the adoption of a spectrum of CR energies.