Letter to the Editor

Low upper limits on the O₂ abundance from the Odin satellite^*

¹ LERMA & FRE 2460 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75140 Paris, France
² Onsala Space Observatory, 439 92 Onsala, Sweden
³ Department of Physics, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
⁴ Laboratoire d'Astronomie Spatiale, BP 8, 13376 Marseille Cedex 12, France
⁵ LESIA, Observatoire de Paris, Section de Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
⁶ LERMA & FRE 2460 du CNRS, École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France
⁷ Herzberg Institute of Astrophysics, NRC of Canada, 5071 W. Saanich Rd, Victoria, BC V9E 2E7, Canada
⁸ Stockholm Observatory, SCFAB, Roslagstullsbacken 21, 106 91 Stockholm, Sweden
⁹ Swedish Space Corporation, PO Box 4207, 171 04 Solna, Sweden
¹⁰ Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada
¹¹ Observatory, PO Box 14, University of Helsinki, 00014 Helsinki, Finland
¹² Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada
¹³ Metsähovi Radio Observatory, Helsinki University of Technology, Otakaari 5A, 02150 Espoo, Finland
¹⁴ Department of Astronomy and Physics, Saint Mary's University, Halifax, Nova Scottia B3H 3C3, Canada
¹⁵ Swedish National Space Board, PO Box 4006, 171 04 Solna, Sweden
¹⁶ CESR, 9 avenue du Colonel Roche, BP 4346, 31029 Toulouse, France

Corresponding author: L. Pagani, laurent.pagani@obspm.fr

Received: 2 December 2002
Accepted: 25 January 2003

Abstract

For the first time, a search has been conducted in our Galaxy for the 119 GHz transition connecting to the ground state of O₂, using the Odin satellite. Equipped with a sensitive 3 mm receiver ( K), 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(H 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. (2002) recently reported a SWAS tentative detection of the 487 GHz transition of O₂ in an outflow wing towards ρ  Oph A in a combination of 7 beams covering approximately . 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 O₂ column density is  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.