Title:
Efficient ortho-para conversion of H2 on interstellar grain surfaces
Authors:
Bron, Emeric; Le Petit, Franck; Le Bourlot, Jacques
Affiliation:
AA(LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Université Paris 06, 92190, Meudon, France emeric.bron@obspm.fr), AB(LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Université Paris 06, 92190, Meudon, France), AC(LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Université Paris 06, 92190, Meudon, France; Université Paris Diderot, Sorbonne Paris Cité, 75013, Paris, France)
Publication:
Astronomy & Astrophysics, Volume 588, id.A27, 17 pp. (A&A Homepage)
Publication Date:
04/2016
Origin:
EDP Sciences
Astronomy Keywords:
ISM: molecules, dust, extinction, photon-dominated region (PDR), astrochemistry, molecular processes, methods: numerical
DOI:
10.1051/0004-6361/201527879
Bibliographic Code:
2016A&A...588A..27B
Abstract
Context. Fast surface conversion between ortho- and para-H2 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 H2 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 H2 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 H2 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.