Volume 569, September 2014
|Number of page(s)||20|
|Published online||30 September 2014|
Surface chemistry in the interstellar medium
II. H2 formation on dust with random temperature fluctuations
LERMA, Observatoire de Paris & CNRS, 5 place Jules Janssen, 92190
2 Université Paris Diderot ; 5 rue Thomas Mann, 75205 Paris, France
Accepted: 3 June 2014
Context. The H2 formation on grains is known to be sensitive to dust temperature, which is also known to fluctuate for small grain sizes due to photon absorption.
Aims. We aim at exploring the consequences of simultaneous fluctuations of the dust temperature and the adsorbed H-atom population on the H2 formation rate under the full range of astrophysically relevant UV intensities and gas conditions.
Methods. The master equation approach is generalized to coupled fluctuations in both the grain’s temperature and its surface population and solved numerically. The resolution can be simplified in the case of the Eley-Rideal mechanism, allowing a fast computation. For the Langmuir-Hinshelwood mechanism, it remains computationally expensive, and accurate approximations are constructed.
Results. We find the Langmuir-Hinshelwood mechanism to become an efficient formation mechanism in unshielded photon dominated region edge conditions when taking those fluctuations into account, despite hot average dust temperatures. It reaches an importance comparable to the Eley-Rideal mechanism. However, we show that a simpler rate equation treatment gives qualitatively correct observable results in full cloud simulations under the most astrophysically relevant conditions. Typical differences are a factor of 2−3 on the intensities of the H2v = 0 lines. We also find that rare fluctuations in cloud cores are sufficient to significantly reduce the formation efficiency.
Conclusions. Our detailed analysis confirms that the usual approximations used in numerical models are adequate when interpreting observations, but a more sophisticated statistical analysis is required if one is interested in the details of surface processes.
Key words: astrochemistry / molecular processes / ISM: molecules / dust, extinction
© ESO, 2014
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