The efficiency of photodissociation for molecules in interstellar ices

TL;DR

This study uses a 1D astrochemical model to determine that the photodissociation rate of molecules in interstellar ices is approximately 30% of that in the gas phase, highlighting the importance of cosmic-ray induced desorption.

Contribution

It provides a quantitative estimate of the solid/gas photodissociation coefficient ratio (~0.3) based on modeling and observational data, improving understanding of interstellar ice chemistry.

Findings

Solid/gas photodissociation coefficient ratio ~0.3

Cosmic-ray induced desorption is crucial in the model

High extinction regions (>22 mag) show uncorrelated observations

Abstract

Processing by interstellar photons affects the composition of the icy mantles on interstellar grains. The rate of photodissociation in solids differs from that of molecules in the gas phase. The aim of this work was to determine an average, general ratio between photodissociation coefficients for molecules in ice and gas. A 1D astrochemical model was utilized to simulate the chemical composition for a line of sight through a collapsing interstellar cloud core, whose interstellar extinction changes with time. At different extinctions, the calculated column densities of icy carbon oxides and ammonia (relative to water ice) were compared to observations. The latter were taken from literature data of background stars sampling ices in molecular clouds. The best-fit value for the solid/gas photodissociation coefficient ratio was found to be ~0.3. In other words, gas-phase photodissociation rate coefficients have to be reduced by a factor of 0.3 before applying them to icy species. A crucial part of the model is a proper inclusion of cosmic-ray induced desorption. Observations sampling gas with total extinctions in excess of ~22 mag were found to be uncorrelated to modelling results, possibly because of grains being covered with non-polar molecules.