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Astron. Astrophys. 324, 221-236 (1997)

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Chemistry and rotational excitation of O [FORMULA] 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]  20) up to C/O = 1. The rate coefficient of the reaction O+OH [FORMULA] 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)

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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