2025ApJ...989...76S

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.