Simulation of proton radiolysis of H2O and O2 ices with the Nautilus code

TL;DR

This study enhances the Nautilus astrochemical code to simulate cosmic ray radiolysis of H2O and O2 ices, analyzing parameter sensitivities and aligning model results with experimental data.

Contribution

It introduces radiolysis processes into Nautilus and investigates their effects on ice chemistry, providing insights into reaction sensitivities and model calibration.

Findings

The model can reproduce experimental abundance ratios with parameter adjustments.

Reducing G-values improves simulation accuracy of H2O destruction rates.

Reaction-diffusion and non-diffusive chemistry significantly influence results under certain conditions.

Abstract

The radiolysis effect of cosmic rays (CRs) plays an important role in the chemistry in molecular clouds. CRs can dissociate the molecules on dust grains, producing reactive suprathermal species and radicals which facilitate the formation of large molecules. We add the radiolysis process and some relevant reactions into the Nautilus astrochemical code. By adjusting some parameters, we investigate the sensitivity of the simulation results of the H2O ice on the removal of reaction-diffusion competition, the removal of non-diffusive chemistry, and the desorption energies of the suprathermal species. We find the model, with a few adjustments of the chemistry, can reproduce the steady-state [H2O2]/[H2O] and [O3]/[O2]_0 abundance ratios in the H2O and O2 radiolysis experiments at any CR flux in the experiments. These adjustments in the model do not fully reproduce the fluence required to reach the steady state. It tends also to overestimate the destruction of H2O as measured in H2O radiolysis experiments. We show that reducing the G-values of H2O radiolysis, which implies an increase in the efficiency of immediate reformation of water locally after ion impact, leads to simulated H2O destruction rates closer to the experiments. The effect of reaction-diffusion competition on the simulation results of H2O ice is significant at $\zeta \lesssim 10^{-14}\ \rm s^{-1}$. The non-diffusive chemistry affects the simulation results at 16 K but not 77K, while the results are sensitive to the desorption energies of suprathermal H, O, O3 and OH at 77 K. Our results show that the steady-state [H2O2]/[H2O] and [O3]/[O2]_0 in experiments can be reproduced by fine-tuning the chemical model, but still call for more 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.