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Astron. Astrophys. 328, 617-627 (1997)

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Chemistry and rotational excitation of O [FORMULA] in interstellar clouds

II. The [FORMULA] O [FORMULA] O isotopomer *

P. Maréchal , Y.P. Viala and L. Pagani

DEMIRM, Observatoire de Paris, 61 Av. de l'Observatoire, F-75014 Paris, France

Received 15 January 1997 / Accepted 9 June 1997

Abstract

Although molecular oxygen O2 is predicted to be an important constituent of the interstellar gas, it has not yet been observed in Galactic interstellar clouds. While ongoing O2 observation projects in our Galaxy have to use stratospheric balloons or satellites, several teams have already tried to observe its isotopomer 16 O18 O with ground-based telescopes. In this paper, a chemical model of interstellar clouds including 18 O and 13 C isotopic reactions with a non-LTE calculation of 16 O18 O rotational population has been used to predict the intensity of the 3 16 O18 O lines observable from the ground, namely the 234, 298 and 402 GHz lines. These predictions are compared to the upper limits obtained by Maréchal et al. (1997a) and to the tentative detection obtained by Pagani et al. (1993) towards L134N. We show that only the 234 GHz line is observable with today's receivers and that the strong upper limits we have reported are not compatible with a standard steady-state model. Clumpiness, high (0.7) C/O ratio or other non-standard models are required to explain our observations. The consequence of the 18 O isotopic fractionation is also discussed.

Key words: molecular processes – ISM: abundances, molecules – radio lines: ISM

* The Table 4 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html

Send offprint requests to: Y. Viala (viala@obspm.fr)
Table 4 only available in electronic form at the CDS (Strasbourg) via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html

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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998

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