Abstract
Context. In spite of the permanent damage suffered from the radiation field (cosmic rays, X-rays, and intense UV-visible radiations), interstellar grains are still covered by ices mantles whose role in interstellar chemistry is well beyond any doubt. This clearly means that the destruction of the ice cover has to be counterbalanced by efficient reconstruction mechanisms. Aims. Our goal is to determine whether the ice, which is still present after irradiation, has a catalytic role in the OH + H2 → H2O + H reaction for its own reconstruction. We focus on the three plausible reaction paths depending on the way reactants OH or H2 are adsorbed at the ice surface. Methods. Calculations were performed in both cluster and solid state approaches, using ab-initio post Hartree-Fock methods for small systems, standard density functional theory (DFT) for larger clusters, and periodic solid state DFT with specific formalisms accounting for weak interactions in systems of infinite dimensions. Results. Although the end product is the same, that is namely the reconstruction of one H2O on the subjacent ice, three different reaction paths are found depending on whether H2 reacts with adsorbed OH(ads), wether OH reacts with adsorbed H2(ads) or wether both OH(ads) and H2(ads) are adsorbed on the ice before reacting. In the first case, there is an activation barrier of ~6 kcal mol−1, requiring the tunneling effect for the reaction to proceed, which is in agreement with preceding studies. In the second case, the reaction is a barrierless process leading to the direct reconstruction of the ice. In the third case, the double adsorption increases the activation barrier due to the lowering of the starting energy. This is found regardless of the dimension of the supporting ice aggregates. Conclusions. Icy grain surfaces play a critical role for their own reconstruction in cold, dense interstellar clouds. The prevalence of tunneling over the direct mechanism should strongly depend on the temperature and local environment.
Highlights
The hypothesis of the existence of water ice in the interstellar medium (ISM) goes back to the beginning of the last century when Eddington (1926) proposed that the water molecules would aggregate to form small grains in the dark clouds between stars
Considering the extremely dilute environment of the ISM, it is agreed that a simple freeze out of the water present in the gas phase cannot be the only origin of the icy grain mantles in which H2O is the dominant component
The problem becomes even more sensitive if one considers that the ice mantles are currently subject to intense damages from the radiation field, resulting in continuous destruction of the ices, which have to be reconstructed one way or another by surface reactions
Summary
The hypothesis of the existence of water ice in the interstellar medium (ISM) goes back to the beginning of the last century when Eddington (1926) proposed that the water molecules would aggregate to form small grains in the dark clouds between stars. A thorough presentation of the history of water formation can be found in Oba et al (2012). Considering the extremely dilute environment of the ISM, it is agreed that a simple freeze out of the water present in the gas phase cannot be the only origin of the icy grain mantles in which H2O is the dominant component. The grain surface is widely covered by H2 which is the dominant species in the environment
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