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

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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


Fast conversion of ortho-[FORMULA] to para-[FORMULA] can occur (and has been observed in the laboratory) on the surface of non magnetic, non metallic grains. In a PDR, the fraction of [FORMULA] converted depends on the ratio of the characteristic time of conversion (of the order of 1 minute) to the residence time of physisorbed [FORMULA]. This later time is itself an exponentially varying function of the grain temperature and of the adsorption binding energy of [FORMULA]. 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 [FORMULA] in the gas phase. Emission lines within para levels of [FORMULA] 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

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000