Context. Water and O₂ are important gas phase ingredients for cooling dense gas when forming stars. On dust grains, H₂O is an important constituent of the icy mantle in which a complex chemistry is taking place, as revealed by hot core observations. The formation of water can occur on dust grain surfaces, and can impact gas phase composition. Aims. The formation of molecules such as OH, H₂O, HO₂ and H₂O ₂, as well as their deuterated forms and O₂ and O ₃ is studied to assess how the chemistry varies in different astrophysical environments, and how the gas phase is affected by grain surface chemistry. Methods. We use Monte Carlo simulations to follow the formation of molecules on bare grains as well as the fraction of molecules released into the gas phase. We consider a surface reaction network, based on gas phase reactions, as well as UV photo-dissociation of the chemical species. Results. We show that grain surface chemistry has a strong impact on gas phase chemistry, and that this chemistry is very different for different dust grain temperatures. Low temperatures favor hydrogenation, while higher temperatures favor oxygenation. Also, UV photons dissociate the molecules on the surface, which can subsequently reform. The formation-destruction cycle increases the amount of species released into the gas phase. We also determine the timescales to form ices in diffuse and dense clouds, and show that ices are formed only in shielded environments, as supported by observations.

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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 H. Furthermore, we aim to find a robust and general surface chemical model that 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 highlighted. Analytic expressions are derived for H, HD, and D formation efficiencies for both types of grains. Rate equations are tested against stochastic methods.Results. As expected, rate equations and stochastic methods diverge for grain sizes below a critical value . For these sizes, D formation decreases to favour HD formation. The formation efficiencies of H 2 and D2 can be calculated by adding a correction factor to the rate equations methods (this factor is a simple exponential factor that becomes unity when a ≥ acrit. We find 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 D is very high compared to models where only physisorption sites are taken into account. When considering a realistic distribution of dust grains, we find that the formation rates of and HD are enhanced by an order of magnitude if small grains are taken into account. The formation of D, on the other hand, comes from the contribution of small (≤ 100 Å) and big (100 ≥Å) grains, depending on the D/H ratio, the grain temperature, and the volume density. The processes described in this paper, which allow a strong enhancement of the deuterated forms of molecular hydrogen, could explain the high degree of deuterium fractionation observed in protostellar environments.@en