Water formation on bare grains: When the chemistry on dust impacts interstellar gas

¹ Kapteyn Astronomical Institute, PO box 800, 9700AV Groningen, The Netherlands
e-mail: cazaux@astro.rug.nl
² LERMA, UMR 8112 du CNRS, Observatoire de Paris et Université de Cergy Pontoise, 5 Mail Gay-Lussac, 95031 Cergy-Pontoise Cedex, France
³ SRON, National Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ School of Physics and Astronomy, University of Leeds, LS2 9JT, Leeds, UK

Received: 8 January 2010
Accepted: 2 July 2010

Abstract

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 H₂O₂, as well as their deuterated forms and O₂ and O₃ 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.