2003A&A...402L..77P

For the first time, a search has been conducted in our Galaxy for the 119GHz transition connecting to the ground state of O₂, using the Odin satellite. Equipped with a sensitive 3 mm receiver (T_sys(SSB)=600K), Odin has reached unprecedented upper limits on the abundance of O₂, especially in cold dark clouds where the excited state levels involved in the 487 GHz transition are not expected to be significantly populated. Here we report upper limits for a dozen sources. In cold dark clouds we improve upon the published SWAS upper limits by more than an order of magnitude, reaching N(O₂)/N(H₂)≤10^–7 in half of the sources. While standard chemical models are definitively ruled out by these new limits, our results are compatible with several recent studies that derive lower O₂ abundances. Goldsmith et al. (2002ApJ...576..814G) recently reported a SWAS tentative detection of the 487GHz transition of O₂ in an outflow wing towards ρ Oph A in a combination of 7 beams covering approximately 10'x14'. In a brief (1.3 hour integration time) and partial covering of the SWAS region (≃65% if we exclude their central position), we did not detect the corresponding 119GHz line. Our 3 sigma upper limit on the O₂ column density is 7.3x10¹⁵.cm^–2. We presently cannot exclude the possibility that the SWAS signal lies mostly outside of the 9' Odin beam and has escaped our sensitive detector.