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

Tian-Yu Tu, Valentine Wakelam, Jean-Christophe Loison, Marin Chabot, Emmanuel Dartois, Yang Chen

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 $ζ\lesssim 10^{-14}\ \rm s^{-1}$. 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.

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

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 . 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.
Paper Structure (26 sections, 24 equations, 8 figures, 4 tables)

This paper contains 26 sections, 24 equations, 8 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Model and Network
  3. Chemical model
  4. Reaction network
  5. Conversion among ion fluence, energy dose, and simulation time
  6. Simulation results
  7. Experimental data of proton-irradiated pure H2O and O2 ice
  8. Pure H2O ice
  9. Pure O2 ice
  10. Results of our best-fit model
  11. Pure H2O ice
  12. Pure O2 ice
  13. Model results using SVHC19 network
  14. Pure H2O ice
  15. Pure O2 ice
  16. ...and 11 more sections

Figures (8)

  • Figure 1: Simulation results of abundance ratios [H2O2]/[H2O] (black lines), [H2]/[H2O] (orange lines), and [O2]/[H2O] (blue lines) with our model (model A) at 16 K (solid lines) and 77 K (dashed lines). The gray shaded and line filled regions show the experimental results of [H2O2]/[H2O] by Gomis_Hydrogen_2004 at 16 and 77 K, respectively, with an assumed uncertainty of a factor of 3.
  • Figure 2: Simulation results of [O3]/$[\ce{O2}]_0$ irradiated by 5 keV protons at 12 K based on model A (solid line) and model S19 (dashed line). The gray shaded region shows the range of steady state [O3]/$[\ce{O2}]_0$ obtained by Ennis_formation_2011 with an uncertainty of a factor of 3. The vertical gray line shows the fluence of $7.6\times 10^{15}\rm \ ions\, cm^{-2}$ which is needed to reach steady state in the experiment of Ennis_formation_2011.
  • Figure 3: Same as Figure \ref{['fig:res_best']} but for model S19.
  • Figure 4: Simulation results of the steady-state [H2O2]/[H2O] abundance ratio in model A as a function of the CR ionization rate. The results at 16 K and 77 K are shown in blue and orange lines, respectively. The solid and dashed lines show the results with and without the reaction-diffusion competition, respectively.
  • Figure 5: Simulation results of the steady-state [O3]/$[\ce{O2}]_0$ abundance ratio in model A as a function of the CR ionization rate.
  • ...and 3 more figures