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#297 new enhancement

H2 formation on grains with temperature fluctuations

Reported by: Gary J. Ferland Owned by: nobody
Priority: good to do Milestone:
Component: atomic/molecular data base Version: trunk
Keywords: Cc:

Description

Title:	
Surface chemistry in the interstellar medium. II. H2 formation on dust with random temperature fluctuations
Authors:	
Bron, Emeric; Le Bourlot, Jacques; Le Petit, Franck
Affiliation:	
AA(LERMA, Observatoire de Paris & CNRS, 5 place Jules Janssen, 92190, Meudon, France; Université Paris Diderot, 5 rue Thomas Mann, 75205, Paris, France jacques.lebourlot@obspm.fr), AB(LERMA, Observatoire de Paris & CNRS, 5 place Jules Janssen, 92190, Meudon, France; Université Paris Diderot, 5 rue Thomas Mann, 75205, Paris, France), AC(LERMA, Observatoire de Paris & CNRS, 5 place Jules Janssen, 92190, Meudon, France)
Publication:	
Astronomy & Astrophysics, Volume 569, id.A100, 20 pp. (A&A Homepage)
Publication Date:	
09/2014
Origin:	
EDP Sciences
Astronomy Keywords:	
astrochemistry, molecular processes, ISM: molecules, dust, extinction
DOI:	
10.1051/0004-6361/201322101
Bibliographic Code:	
2014A&A...569A.100B
Abstract

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.

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