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Astron. Astrophys. 324, 221-236 (1997)
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Chemistry and rotational excitation of O ![[FORMULA]](img1.gif)
in interstellar clouds
I. Predicted emissivities of lines for the ODIN, SWAS, PRONAOS-SMH and PIROG 8 submillimeter receivers
P. Maréchal 1,
Y.P. Viala 1 and
J.J. Benayoun 2
1 Observatoire de Paris, DEMIRM, URA 336 du CNRS, 61 Av. de
l'Observatoire, F-75014 Paris, France
2 Observatoire de Grenoble, Laboratoire d'Astrophysique,
Université Joseph Fourier, B.P.53X, F-38041 Grenoble Cedex,
France
Received 27 September 1996 / Accepted 2 January 1997
Abstract
Molecular oxygen has not yet been observed in interstellar clouds
because of the opacity of the Earth's atmosphere. Due to its potential
importance in interstellar chemistry, several projets attempting to
detect rotational lines of O2 are being developed using
millimeter and submillimeter receivers embarked on satellites or
stratospheric balloons: ODIN for the 119 and 487 GHz lines, SWAS
for the 487 GHz line, PRONAOS-SMH for the 368 GHz line and
PIROG 8 for the 425 GHz line. As a theoretical preparation
to these projects and taking advantage of recent developments in
interstellar chemistry as well as recent cross-section calculations of
collisional excitation of O2, we have used an interstellar
cloud model to perform a non-LTE calculation of O2
rotational population. O2 column densities and emissivities
of its rotational lines at (sub)millimeter wavelengths are predicted
for various conditions in diffuse, translucent and dense dark clouds,
covering a range of visual extinction from 1 to 30. The effects of
density, temperature, external ultraviolet radiation field and gas
phase elemental abundances on the O2 abundance and
rotational excitation have been investigated. Our results are
confronted to the ones obtained by Black & Smith (1984) who
adressed initially the problem of the detectability of interstellar
O2. If density and temperature have little influence on the
O2 abundance, it is not the case for the UV radiation field
which efficiently destroys O2 so as to prevent its
detection in most clouds as soon as it is enhanced by a factor of 1000
with respect to the local standard value. The most drastic parameter
that influences the abundance of O2 - as well as that of OH
and H2 O - is the gas-phase C/O abundance ratio: for C/O
ratio larger than 0.7, O2 becomes unobservable by all
forthcoming missions except for the 119 GHz line which remains
observable in very opaque clouds (A ![[FORMULA]](img2.gif)
20) up
to C/O = 1. The rate coefficient of the reaction O+OH
![[FORMULA]](img3.gif)
O2 +H which produces molecular oxygen
has little influence on the O2 abundance in clouds
sufficiently opaque to allow detection; it however controls the OH
abundance since it is the main destruction process of this molecule.
Our model calculations also predict that the radiative de-excitation
of O2 rotational levels could be an important cooling agent
in cold molecular clouds with an efficiency comparable to that of
CO.
Key words: molecular
processes 
ISM: abundances,
molecules
radio lines: ISM
Send offprint requests to: P. Maréchal (Priscilla.Marechal@obspm.fr)
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998
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