Chemistry and rotational excitation of O_2 in interstellar clouds. II. The 16O^18O isotopomer

Although molecular oxygen O_2 is predicted to be an important constituent of the interstellar gas, it has not yet been observed in Galactic interstellar clouds. While ongoing O_2 observation projects in our Galaxy have to use stratospheric balloons or satellites, several teams have already tried to observe its isotopomer (16) O(18) 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) O(18) O rotational population has been used to predict the intensity of the 3 (16) O(18) O lines observable from the ground, namely the 234, 298 and 402 GHz lines. These predictions are compared to the upper limits obtained by Marechal 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. 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