Simulation of proton radiolysis of H 2 O and O 2 ices with the Nautilus code

Context . The radiolysis effect of cosmic rays (CRs) plays an important role in the chemistry of molecular clouds. Cosmic rays can dissociate molecules on dust grains, producing reactive suprathermal species and radicals, which facilitate the formation of large molecules. Aims . In this study, we attempt to reproduce laboratory experimental results on the radiolysis of pure H 2 O and O 2 ice using the Nautilus astrochemical code, and evaluate the effects of varying several uncertain physical and chemical parameters. Methods . We added radiolysis reactions, quenching reactions of suprathermal species, and reactions between suprathermal and thermal species to the Nautilus code. By adjusting the parameters in the code, we investigated the sensitivity of the simulation results for H 2 O ice to the removal of reaction-diffusion competition, the removal of non-diffusive chemistry, and the desorption energies of suprathermal species. Results . We find that the Nautilus model, with a few adjustments to the chemistry, reproduces the steady-state [H 2 O 2 ]/[H 2 O] and [O 3 ]/[O 2 ] 0 abundance ratios in the H 2 O and O 2 radiolysis experiments at any CR flux explored. However, these adjustments do not fully reproduce the fluence required to reach the steady state. The model also tends to overestimate the destruction of H 2 O measured in H 2 O radiolysis experiments. We show that reducing the G -values of H 2 O radiolysis, which implies an increase in the efficiency of immediate local reformation of water molecules after impact by ions, leads to simulated H 2 O destruction rates that are closer to experimental results. The effect of reaction-diffusion competition on the simulation results for H 2 O ice is significant only at ζ ≲ 10 −14 s −1 . Non-diffusive chemistry affects the simulation results at 16 K but not at 77 K, while the results are sensitive to the desorption energies of suprathermal H, O, O 3 and OH at 77 K. Conclusions . Our results show that the steady-state [H 2 O 2 ]/[H 2 O] and [O 3 ]/[O 2 ] 0 in radiolysis experiments can be reproduced by fine-tuning the chemical model, but still call for further constraints on the intermediate pathways in the radiolysis processes, especially the ion chemistry in the ice bulk, as well as activation barriers and branching ratios of the reactions in the network.