The role of carbon grains in the deuteration of H2

TL;DR

This study models the surface chemistry of carbonaceous dust grains to understand their role in the formation and deuteration of molecular hydrogen, revealing significant effects of grain size and chemisorbed sites on deuterium enrichment.

Contribution

It introduces a comprehensive chemical model including chemisorption effects and compares rate equations with stochastic methods for H2 and D2 formation on carbon grains.

Findings

D2 formation decreases for small grains, favoring HD formation.

Chemisorbed sites significantly enhance HD and D2 formation efficiencies.

Including small grains increases overall H2 and HD formation rates by an order of magnitude.

Abstract

Aims: The production of molecular hydrogen and its deuterated forms onto carbonaceous dust grains is investigated in detail. The goal of this study is to estimate the importance of the chemistry occuring on grain surfaces for the deuteration of H2. Furthermore, we aim to find a robust and general surface chemical model which can be used in different astrophysical environments. Methods: Surface processes are described for the cases of graphitic and amorphous--carbon grains, where laboratory work is available. Langmuir--Hinshelwood as well as Eley--Rideal surface chemistries are included in the model and their relative contributions are highlighted. Analytic expressions are derived for H2, HD, and D2 formation efficiencies for both type of grains. Rate equations are tested against stochastic methods. Results: As expected, rate equations and stochastic methods diverge for grain sizes lower than a critical value acrit. For grain sizes below this critical value, D2 formation decreases to favour HD formation. The formation efficiencies of H2 and D2 can be calculated by adding a correction factor to the rate equations methods. We found that because of the presence of chemisorbed sites, which can store atoms to form molecules up to high grain temperatures, the formation efficiency of HD and D2 is very high compared to models where only physisorption sites are taken into account. When considering a realistic distribution of dust grains, we found that the formation rate of H2 and HD is enhanced by an order of magnitude if small grains are taken into account. The processes described in this paper, that allow a strong enhancement of the deuterated forms of molecular hydrogen, could explain the high degree of deuterium fractionation observed in protostellar environments.