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A&A 482, 849-853 (2008)
DOI: 10.1051/0004-6361:200809436

Odin observations of the Galactic centre in the 118-GHz band

Upper limit to the O2 abundance
Aa. Sandqvist1, B. Larsson1, Å. Hjalmarson2, P. Bergman2, P. Bernath3, U. Frisk4, M. Olberg2, L. Pagani5, and L. M. Ziurys6

1  Stockholm Observatory, AlbaNova University Center, 106 91 Stockholm, Sweden
    e-mail: aage@astro.su.se
2  Onsala Space Observatory, 439 92 Onsala, Sweden
3  Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
4  Swedish Space Corporation, PO Box 4207, 171 04 Solna, Sweden
5  LERMA & UMR8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
6  Arizona Radio Observatory, Steward Observatory, University of Arizona, Tucson, AZ 85721, USA

(Received 22 January 2008 / Accepted 23 February 2008)

Abstract
Aims. The Odin satellite has been used to search for the 118.75-GHz line of molecular oxygen (O2) in the Galactic centre.
Methods. Odin observations were performed towards the Sgr $\rm $ circumnuclear disk (CND), and the Sgr A +20 km s-1 and +50 km s-1 molecular clouds using the position-switching mode. Supplementary ground-based observations were carried out in the 2-mm band using the ARO Kitt Peak 12-m telescope to examine suspected SiC features.
Results. A strong emission line was found at 118.27 GHz, attributable to the J = 13-12 HC3N line. Upper limits are presented for the 118.75-GHz O2 (11-10) ground transition line and for the 118.11-GHz $^{3}\Pi_{2}$, J = 3-2 ground state SiC line at the Galactic centre. Upper limits are also presented for the 487-GHz O2 line in the Sgr A +50 km s-1 cloud and for the 157-GHz, J = 4-3, SiC line in the Sgr A +20 and +50 km s-1 clouds, as well as the CND. The CH3OH line complex at 157.2-157.3 GHz has been detected in the +20 and +50 km s-1 clouds but not towards Sgr $\rm $/CND.
Conclusions. A $3\sigma$ upper limit for the fractional abundance ratio of [O2] /[H2] is found to be X(O2) $\le$ 1.2 $\times$ 10-7 towards the Sgr A molecular belt region.


Key words: Galaxy: center -- ISM: individual objects: Sgr A -- ISM: molecules -- ISM: clouds -- ISM: abundances



© ESO 2008