Low upper limits on the O 2 abundance from the Odin satellite

For the first time, a search has been conducted in our Galaxy for the 119 GHz transition connecting to the ground state of O 2, using the Odin satellite. Equipped with a sensitive 3 mm receiver (T sys(SSB) = 600 K), Odin has reached unprecedented upper limits on the abundance of O 2, 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(O 2)/N(H 2) ≤ 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 O 2 abundances. Goldsmith et al. (2002) recently reported a SWAS tentative detection of the 487 GHz transition of O 2 in an outflow wing towards ρ Oph A in a combination of 7 beams covering approximately 10′ × 14′. 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 119 GHz line. Our 3 sigma upper limit on the O 2 column density is 7.3 × 10 15 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.link_to_subscribed_fulltex