Effect of surface H$_2$ on molecular hydrogen formation on interstellar grains

TL;DR

This study examines how surface H$_2$ molecules influence molecular hydrogen formation efficiency on interstellar grains, revealing that the effect varies with diffusion barriers and cloud conditions, with implications for astrochemical models.

Contribution

It introduces detailed Monte Carlo simulations to analyze the impact of surface H$_2$ on H$_2$ formation, highlighting the dependence on diffusion barriers and cloud environment.

Findings

H$_2$ presence can reduce H atom recombination efficiency in diffuse clouds.

Recombination efficiency varies from nearly 0 to above 0.5 depending on model parameters.

H$_2$ formation efficiency in translucent clouds is sensitive to H$_2$ diffusion barriers.

Abstract

We investigate how the existence of hydrogen molecules on grain surfaces may affect H$_2$ formation efficiency in diffuse and translucent clouds. Hydrogen molecules are able to reduce the desorption energy of H atoms on grain surfaces in models. The detailed microscopic Monte Carlo method is used to perform model simulations. We found that the impact of the existence of H$_2$ on H$_2$ formation efficiency strongly depends on the diffusion barriers of H$_2$ on grain surfaces. Diffuse cloud models that do not consider surface H$_2$ predict that H atom recombination efficiency is above 0.5 over a grain temperature (T) range 10 K and 14 K. The adopted H$_2$ diffusion barriers in diffuse cloud models that consider surface H$_2$ are 80$\%$ H$_2$ desorption energies so that H$_2$ can be trapped in stronger binding sites. Depending on model parameters, these diffuse cloud models predict that the recombination efficiency is between nearly 0 and 0.5 at 10 K $\leq$ T $\leq$ 14 K. Translucent cloud model results show that H$_2$ formation efficiency is not affected by the existence of surface H$_2$ if the adopted average H$_2$ diffusion barrier on grain surfaces is low (194 K) so that H$_2$ can diffuse rapidly on grain surfaces. However, the recombination efficiency can drop to below 0.002 at T $\geq$ 10 K if higher average H$_2$ diffusion barrier is used (255 K) in translucent cloud models.