Modeling the UV-photon irradiation of CS₂-bearing ices in the laboratory with the pyRate gas-grain astrochemical code. New insights into the missing sulfur problem

Observations indicate that the total abundance of S-bearing species in dense clouds is orders of magnitude lower than the cosmic sulfur abundance. Addressing this ``missing sulfur problem'' requires a combination of astronomical observations, laboratory experiments, and theoretical models. For this study, we used the pyRate astrochemical model to simulate the vacuum-ultraviolet-photon irradiation of a CO₂: CS₂ ice mixture at 10 K in the laboratory, with the goal of supporting the interpretation of the experimental results and testing our current understanding of the sulfur evolution in interstellar ices. For this purpose, the astrochemical model was adapted to the experimental conditions, and the chemical network was compiled from several sources to ensure that all known reactions involving sulfur species were included. The results indicate that non-diffusive chemistry is necessary to reproduce the formation of S-bearing species observed in the experiment. However, some discrepancies were found in the major S-bearing ice chemistry products predicted by the model and the experiment. The compounds OCS, CS, and SO are overpredicted by the model, but it falls short in accounting for ̊m SO₂ and sulfur allotropes. These discrepancies are likely due to a combination of incomplete knowledge on the chemical reactions at play (either because of missing reactions and/or because of unconstrained reaction barriers), and uncertainties in the experimental analysis. This work represents the first effort to model the chemistry of a multicomponent ice analog with a rate-equation-based code, and highlights the complementary nature of theoretical and experimental astrochemistry to disentangle the chemical evolution of sulfur in the interstellar medium.