Astron. Astrophys. 360, 656-662 (2000)

Ortho to para conversion of H₂ on interstellar grains

J. Le Bourlot

DAEC, Observatoire de Paris and Université Paris 7, UFR de Physique

Received 28 April 2000 / Accepted 15 June 2000

Abstract

Fast conversion of ortho- to para- can occur (and has been observed in the laboratory) on the surface of non magnetic, non metallic grains. In a PDR, the fraction of converted depends on the ratio of the characteristic time of conversion (of the order of 1 minute) to the residence time of physisorbed . This later time is itself an exponentially varying function of the grain temperature and of the adsorption binding energy of . Conversion is very efficient in cool PDRs if the dust temperature is below a critical temperature that we determine, and leads to a much reduced ortho to para ratio of in the gas phase. Emission lines within para levels of are significantly enhanced.

Key words: molecular processes ISM: molecules ISM: abundances ISM: dust, extinction

Send offprint requests to: J. Le Bourlot

Correspondence to: Jacques.Lebourlot@obspm.fr

This article contains no SIMBAD objects.

Contents

• 1. Introduction
• 2. Physical parameters
  • 2.1. Sticking coefficient
  • 2.2. Binding energy
  • 2.3. Conversion time scale
  • 2.4. Grain temperature
  • 2.5. Conversion efficiency
  • 2.6. Other desorption mechanisms
• 3. Models
  • 3.1. Influence of the binding energy
  • 3.2. Influence of the radiation field
• 4. Comparison with observations
• 5. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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