EDP Sciences
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Volume 388, Number 1, June II 2002
Page(s) 346 - 354
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20020489

A&A 388, 346-354 (2002)
DOI: 10.1051/0004-6361:20020489

Surface hydrogenation in diffuse and translucent interstellar clouds

P. T. O'neill, S. Viti and D. A. Williams

University College London, Gower Street, London WC1E 6BT, UK

(Received 15 March 2001 / Accepted 28 March 2002 )

We investigate the effects of grain-surface hydrogenation reactions on gas phase chemical models of diffuse and translucent clouds in the interstellar medium. Models in which gas phase species freeze out on to dust without release do not match observed column densities well. Expanding on previous work we extend our models to include the release of C, N, O, S and CO following hydrogenation on the grain surface. The results show that such mechanisms do improve the ability of chemical models to reproduce observed abundances, not only through the release of otherwise frozen-out species but also because of the additional hydrogenating reactions. For example, the predicted column densities of $\ensuremath{\rm {NH}} $ in diffuse clouds and $\ensuremath{\rm {H_2S}} $ and $\ensuremath{\rm {NH_3}} $ in translucent clouds match observations better in models with grain-surface hydrogenation.

Key words: astrochemistry -- molecular processes -- ISM: abundances -- ISM: molecules

Offprint request: S. Viti, sv@star.ucl.ac.uk

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