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| - Efficient ortho-para conversion of H 2 on interstellar grain surfaces
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| - Context. Fast surface conversion between ortho- and para-H 2 has been observed in laboratory studies, and it has been proposed that this mechanism plays a role in the control of the ortho-para ratio in the interstellar medium. Observations of rotational lines of H 2 in photo-dissociation regions (PDRs) have indeed found significantly lower ortho-para ratios than expected at equilibrium. The mechanisms controlling the balance of the ortho-para ratio in the interstellar medium thus remain incompletely understood, while this ratio can affect the thermodynamical properties of the gas (equation of state, cooling function). Aims. We aim to build an accurate model of ortho-para conversion on dust surfaces based on the most recent experimental and theoretical results, and to validate it by comparison to observations of H 2 rotational lines in PDRs. Methods. We propose a statistical model of ortho-para conversion on dust grains with fluctuating dust temperatures. It is based on a master equation approach. This computation is then coupled to full PDR models and compared to PDR observations. Results. We show that the observations of rotational H 2 lines indicate a high conversion efficiency on dust grains and that this high efficiency can be accounted for if taking dust temperature fluctuations into account with our statistical model of surface conversion. Simpler models that neglect the dust temperature fluctuations do not reach the high efficiency deduced from the observations. Moreover, this high efficiency induced by dust temperature fluctuations is very insensitive to the values of the model’s microphysical parameters. Conclusions. Ortho-para conversion on grains is thus an efficient mechanism in most astrophysical conditions and can play a significant role in controlling the ortho-para ratio.
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