Chemistry on rotating grain surfaces: ro-thermal hopping and segregation of molecules in ice mantles

Abstract: Grain surfaces play a central role in the formation and desorption of molecules in space. To form molecules on a grain surface, adsorbed species trapped in binding sites must be mobile and migrate to adjacent sites. Thermal hopping is a popular mechanism for the migration of adsorbed species when the grain surface is warmed up by stellar radiation. However, previous studies disregarded the fact that grains can be spun-up to suprathermal rotation by radiative torques (RATs) during grain heating process. To achieve an accurate model of surface astrochemistry, in this paper, we study the effect of grain suprathermal rotation by RATs on thermal hopping of adsorbed species on icy grain mantles. We find that centrifugal force due to grain suprathermal rotation can increase the mobility of radicals on/in the ice mantle compared to the prediction by thermal hopping, and we term this mechanism ro-thermal hopping. The rate of ro-thermal hopping depends both on the local radiation energy density (i.e., grain temperature) and gas density, whereas thermal hopping only depends on grain temperature. We calculate the decrease in grain temperature required by ro-thermal hopping to produce the same hopping rate as thermal hopping and find that it increases with increasing the diffusion energy and decreasing the gas density. We finally study the effect of grain suprathermal rotation on the segregation of ice mixtures and find that ro-thermal segregation of CO$_2$ from H$_2$O-CO$_2$ ices can occur at much lower temperatures than thermal segregation reported by experiments. Our results indicate that grain suprathermal rotation can enhance mobility, formation, desorption, and segregation of molecules in icy grain mantles.