The Effect of Weak Cosmic Ray Heating Events on the Desorption of $\rm H_2$

TL;DR

This study models the impact of weak cosmic ray-induced heating events on the desorption of molecular hydrogen from interstellar dust grains, finding that such weak events are insufficient for significant H2 desorption but influence other species at low densities.

Contribution

The paper introduces a numerical desorption model that includes weak cosmic ray heating events, extending previous models focused only on strong heating events, to better understand H2 desorption processes.

Findings

Weak cosmic ray heating events do not cause significant H2 desorption.

Weak heating affects abundances of lightly-bound species at low column densities.

Further research needed on grain size distribution effects.

Abstract

The typical amount of molecular hydrogen (${\rm H_2}$) in interstellar ices is not known, but significant freeze-out of ${\rm H_2}$ on dust grains is not expected. However, chemical models ubiquitously predict large amounts of $\rm H_2$ freeze-out in dense cloud conditions, and specialized treatments are needed to control the $\rm H_2$ population on grains. Here we present a numerical desorption model where the effect of weak heating events induced by cosmic rays (CRs) that heat grains to temperatures of a few tens of Kelvin at high frequencies is included, improving upon earlier desorption models that only consider strong heating events (maximum grain temperature close to 100 K) that occur at a low frequency. A temperature of a few tens of Kelvin is high enough to induce efficient desorption of $\rm H_2$, but we find that even the weak heating events do not occur often enough to lead to significant $\rm H_2$ desorption. Taking the weak heating events into account does affect the predicted abundances of other lightly-bound species, but the effect is restricted to low column densities. We make here the canonical assumption that the grains are spherical with a radius of 0.1 $\mu$m. It is conceivable that in the case of a grain size distribution, weak heating events could provide a boost to $\rm H_2$ desorption coming off small grains, which are the most numerous. Further studies are still required to better quantify the role of CRs in the desorption of $\rm H_2$ and other weakly bound species.