On simulating the proton-irradiation of O₂ and H₂O ices using astrochemical-type models, with implications for bulk reactivity

Many current astrochemical models explicitly consider the species that comprise the bulk of interstellar dust grain ice mantles separately from those in the top few monolayers. Bombardment of these ices by ionizing radiation—whether in the form of cosmic rays, stellar winds, or radionuclide emission—represents an astrochemically viable means of driving a rich chemistry even in the bulk of the ice mantle, now supported by a large body of work in laboratory astrophysics. In this study, using an existing rate-equation-based astrochemical code modified to include a method of considering radiation chemistry recently developed by us, we attempted to simulate two such studies in which (a) pure O2 ice at 5 K and (b) pure H2O ice at 16 K and 77 K, were bombarded by keV H+ ions. Our aims were twofold: (1) to test the capability of our newly developed method to replicate the results of ice-irradiation experiments, and (2) to determine how bulk chemistry in such a well-constrained system is best handled using the same gas-grain codes that are used to model the interstellar medium. We found that our modified astrochemical model was able to reproduce both the abundance of O3 in the 5 K pure O2 ice, as well as both the abundance of H2O2 in the 16 K water ice and the previously noted decrease of hydrogen peroxide at higher temperatures. However, these results require the assumption that radicals and other reactive species produced via radiolysis react quickly and non-diffusively with neighbors in the ice