Journal Article

The Effect of Weak Cosmic-ray Heating Events on the Desorption of H2

Silsbee, Kedron, Padovani, Marco, Carbajal, Naomi, Sipilä, Olli, Caselli, Paola

2025

Overview

The typical amount of molecular hydrogen (H2) in interstellar ices is not known, but significant freeze-out of H2 on dust grains is not expected. However, chemical models ubiquitously predict large amounts of H2 freeze-out in dense cloud conditions, and specialized treatments are needed to control the H2 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 H2, but we find that even the weak heating events do not occur often enough to lead to significant H2 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 μm. It is conceivable that in the case of a grain size distribution, weak heating events could provide a boost to H2 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 H2 and other weakly bound species.

• Publisher: IOP Publishing
• Subject: Cosmic ray showers / Cosmic rays / Desorption / Grain size distribution / Heating / Ice / Interstellar chemistry / Interstellar ices / Interstellar matter