Towards a better understanding of ice mantle desorption by cosmic rays

TL;DR

This study refines models of cosmic ray-induced ice mantle desorption, emphasizing the importance of sporadic, grain size-dependent processes, which significantly increase desorption rates and impact interstellar chemistry.

Contribution

It introduces a detailed, sporadic desorption model considering grain size and co-desorption, challenging the continuous approximation used previously.

Findings

Desorption rates are likely much higher than previously estimated.

Small grains dominate the desorption process.

Desorption can cause significant chemical enrichment in dark interstellar environments.

Abstract

The standard model of cosmic ray heating-induced desorption of interstellar ices is based on a continuous representation of the sporadic desorption of ice mantle components from classical (0.1 micron) dust grains. This has been re-evaluated and developed to include tracking the desorption through (extended) grain cooling profiles, consideration of grain size-dependencies and constraints to the efficiencies. A model was then constructed to study the true, sporadic, nature of the process with possible allowances from species co-desorption and whole mantle desorption from very small grains. The key results from the study are that the desorption rates are highly uncertain, but almost certainly significantly larger than have been previously determined. For typical interstellar grain size distributions it is found that the desorption is dominated by the contributions from the smallest grains. The sporadic desorption model shows that, if the interval between cosmic ray impacts is comparable to, or less than, the freeze-out timescale, the continuous representation is inapplicable; chemical changes may occur on very long timescales, resulting in strong gas phase chemical enrichments that have very non-linear dependences on the cosmic ray flux. The inclusion of even limited levels of species co-desorption and/or the contribution from very small grains further enhances the rates, especially for species such as H2O. In general we find that cosmic-ray heating is the dominant desorption mechanism in dark environments. These results may have important chemical implications for protostellar and protoplanetary environments.