Incorporation of stochastic chemistry on dust grains in the Meudon PDR code using moment equations

F. Le Petit*, B. Barzel, O. Biham, E. Roueff, J. Le Bourlot

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

26 Scopus citations


Context. Unlike gas-phase reactions, chemical reactions taking place on interstellar dust grain surfaces cannot always be modeled by rate equations. Because of the small grain sizes and low flux, these reactions may exhibit large fluctuations and thus require stochastic methods such as the moment equations.Aims. We evaluate the formation rates of H2, HD, and D 2 molecules on dust grain surfaces and their abundances in the gas phase under interstellar conditions.Methods. We incorporate the moment equations into the Meudon PDR code and compare the results with those obtained from the rate equations.Results. We find that within the experimental constraints on the energy barriers for both diffusion and desorption and the density of adsorption sites on the grain surface, H2, HD and D2 molecules can be formed efficiently on dust grains.Conclusions. In a wide range of conditions, the moment equation results agree with those obtained from the rate equations. However, for a range of relatively high grain temperatures, there are significant deviations: the rate equations fail, while the moment equations provide accurate results. The incorporation of the moment equations into the PDR code can be extended to other reactions taking place on grain surfaces.

Original languageAmerican English
Pages (from-to)1153-1165
Number of pages13
JournalAstronomy and Astrophysics
Issue number3
StatePublished - 3 Oct 2009


  • Astrochemistry
  • ISM: clouds
  • ISM: general
  • ISM: molecules


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