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

TL;DR

This paper introduces a new dynamic model for cosmic-ray-induced desorption of interstellar ices, accounting for ice content, and demonstrates its impact on molecular abundances in starless and prestellar cores.

Contribution

The study presents a revised CRD model that dynamically links desorption efficiency to ice content, improving upon previous fixed-time models.

Findings

Dynamic CRD reduces gas-phase molecular abundances by up to an order of magnitude.

Ice abundances are largely unaffected by variations in grain cooling time.

Results are similar to static models when only one ice monolayer is active.

Abstract

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) sublimation of the ice. Current cosmic-ray-induced desorption (CRD) models assume 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 cloud envelopes. We find that inside starless and prestellar cores, introducing dynamic CRD can decrease gas-phase abundances by up to an order of magnitude in two-phase chemical models. In three-phase chemical models our model produces very similar results to the static cooling scheme - when only one monolayer of ice is considered active. Ice abundances are generally insensitive to variations in the grain cooling time. 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.