Towards a better understanding of ice mantle desorption by cosmic rays

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 ($0.1\, \mu$m) 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.

A Solution for the Density Dichotomy Problem of Kuiper Belt Objects with Multispecies Streaming Instability and Pebble Accretion

“…When the dust-to-gas ratio of these submicron grain species decreases to 10 −4 , typical of Class I-II disks, the gas becomes optically thin to UV as close to the midplane as 1.5 scale heights, a layer where even low levels of turbulence would suffice to loft sub-millimetersized grains into. Bulk motions due to the VSI would make it even easier to get these particles to the UV-irradiated layer, and cosmic-ray desorption (Silsbee et al 2021;Sipilä et al 2021;Rawlings 2022) would further enhance the loss of ice coating from grains. As a result of these processes, smaller grains should have less ice than larger grains that reside near the disk midplane, shielded from UV and cosmic rays.…”