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Astron. Astrophys. 328, 617-627

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Chemistry and rotational excitation of O2 in interstellar clouds

II. The 16O18O isotopomer1

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


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 16O18O with ground-based telescopes. In this paper, a chemical model of interstellar clouds including 18O and 13C isotopic reactions with a non-LTE calculation of 16O18O rotational population has been used to predict the intensity of the 3 16O18O 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 18O isotopic fractionation is also discussed.

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

1Table 4 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( or via http://cdsweb.u-strasbg.fr/Abstract.html

Send offprint requests to: Y. Viala (viala@obspm.fr)

© European Southern Observatory (ESO) 1997

Online publication: November 24, 1997
Last change: March 26, 1998