Odin observations of the Galactic centre in the 118-GHz band*
Upper limit to the O2 abundance
Stockholm Observatory, AlbaNova University Center, 106 91 Stockholm, Sweden e-mail: firstname.lastname@example.org
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
Accepted: 23 February 2008
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 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 HC3N line. Upper limits are presented for the 118.75-GHz O2 ground transition line and for the 118.11-GHz , 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, , 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 /CND.
Conclusions. A upper limit for the fractional abundance ratio of [ O2] /[ H2] is found to be X(O2) ≤ 1.2 10-7 towards the Sgr A molecular belt region.
Key words: Galaxy: center / ISM: individual objects: Sgr A / ISM: molecules / ISM: clouds / ISM: abundances
Based on observations with Odin, a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB), the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes) and Centre National d'Études Spatiales (CNES). The Swedish Space Corporation was the industrial prime contractor and is also responsible for the satellite operation.
© ESO, 2008