Free access article

A&A 504, L5-L8 (2009)
DOI: 10.1051/0004-6361/200912746

Letter

Accurate rate coefficients for models of interstellar gas-grain chemistry

I. Lohmar¹, J. Krug¹, and O. Biham<sup>2

1</sup>  Institute for Theoretical Physics, University of Cologne, Cologne, Germany
    e-mail: il@thp.uni-koeln.de
²  Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel

Received 22 June 2009 / Accepted 28 July 2009

Abstract
Aims. The methodology for modeling grain-surface chemistry has been greatly improved by taking into account the grain size and fluctuation effects. However, the reaction rate coefficients currently used in all practical models of gas-grain chemistry are inaccurate by a significant amount. We provide expressions for these crucial rate coefficients that are both accurate and easy to incorporate into gas-grain models.
Methods. We use exact results obtained in earlier work, where the reaction rate coefficient was defined by a first-passage problem, which was solved using random walk theory.
Results. The approximate reaction rate coefficient presented here is easy to include in all models of interstellar gas-grain chemistry. In contrast to the commonly used expression, the results that it provides are in perfect agreement with detailed kinetic Monte Carlo simulations. We also show the rate coefficient for reactions involving multiple species.

Key words: astrochemistry -- ISM: clouds -- ISM: molecules -- dust, extinction -- molecular processes

© ESO 2009

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