Astronomy & Astrophysics

Free access article

A&A 402, L77-L81 (2003)
DOI: 10.1051/0004-6361:20030344

Letter

Low upper limits on the O $\mathsf$ abundance from the Odin satellite

L. Pagani¹, A. O. H. Olofsson², P. Bergman², P. Bernath³, J. H. Black², R. S. Booth², V. Buat⁴, J. Crovisier⁵, C. L. Curry³, P. J. Encrenaz¹, E. Falgarone⁶, P. A. Feldman⁷, M. Fich³, H. G. Floren⁸, U. Frisk⁹, M. Gerin⁶, E. M. Gregersen¹⁰, J. Harju¹¹, T. Hasegawa¹², Å. Hjalmarson², L. E. B. Johansson², S. Kwok¹², B. Larsson⁸, A. Lecacheux⁵, T. Liljeström¹³, M. Lindqvist², R. Liseau⁸, K. Mattila¹¹, G. F. Mitchell¹⁴, L. H. Nordh¹⁵, M. Olberg², G. Olofsson⁸, I. Ristorcelli¹⁶, Aa. Sandqvist⁸, F. von Scheele⁹, G. Serra¹⁶, N. F. Tothill¹⁴, K. Volk¹², T. Wiklind² and C. D. Wilson¹⁰

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

(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 O ₂, using the Odin satellite. Equipped with a sensitive 3 mm receiver ( $T_{\rm sys}({\rm SSB}) = 600$ 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 $_2) \leq 10^$ 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 $\rho$ Oph A in a combination of 7 beams covering approximately $10\arcmin \times 14\arcmin$ . In a brief (1.3 hour integration time) and partial covering of the SWAS region ( $\approx$ 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 $7.3\times 10^$ 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.

Key words: radio lines: ISM -- ISM: molecules -- Galaxy: abundances

Offprint request: L. Pagani, laurent.pagani@obspm.fr

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Letter

Low upper limits on the O abundance from the Odin satellite

Low upper limits on the O $\mathsf$ abundance from the Odin satellite