Cosmic-ray-induced dissociation and reactions in warm interstellar ices

TL;DR

Cosmic rays can induce heating, dissociation, and reactions in interstellar ices, with dissociation being the most significant process, leading to increased complex molecule formation in star-forming regions.

Contribution

This study quantitatively evaluates the impact of cosmic-ray-induced processes on ice chemistry in star-forming cores using a detailed chemical kinetics model.

Findings

Cosmic-ray effects mainly increase carbon-chain species abundance.

Major species abundances are affected by only a few percentage points.

Dissociation significantly influences complex molecule synthesis.

Abstract

Context. Cosmic ray particles that hit interstellar grains in dark molecular cores may induce whole-grain heating. The high temperature of a CR-heated grain allows energy barriers for bulk diffusion and reactions to be overcome. Additionally, ice molecules are destroyed by direct cosmic-ray induced dissociation. Aims. We provide a justified estimate of the significance of cosmic-ray induced surface-mantle diffusion, chemical reactions in ice, and dissociation of ice species in a star-forming interstellar cloud core. Methods. We considered a gas clump in a collapsing low-mass prestellar core and during the initial stages of protostellar envelope heating with a three-phase chemical kinetics model. The model includes a proper treatment of the stochastic aspect of whole-grain heating and new experimental data for dissociation. Results. We found that the cosmic-ray-induced effects are mostly limited to an increase in abundance for carbon-chain species. The effect on major species abundances is a few percentage points at most. The HNC to HCN ice abundance ratio in ice is increased. Conclusions. Among the processes considered in the model, dissociation is probably the most significant, while diffusion and reactions on warm grains are less important. All three processes facilitate the synthesis of complex molecules, including organic species.