2008A&A...482..849S

The Odin satellite has been used to search for the 118.75-GHz line of molecular oxygen (O₂) in the Galactic centre. Odin observations were performed towards the Sgr A^* circumnuclear disk (CND), and the Sgr A +20km/s and +50km/s 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. A strong emission line was found at 118.27GHz, 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 ³Π₂, 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 +50km/s cloud and for the 157-GHz, J=4-3, SiC line in the Sgr A +20 and +50km/s clouds, as well as the CND. The CH₃OH line complex at 157.2-157.3GHz has been detected in the +20 and +50 km/s clouds but not towards Sgr A^*/CND. A 3σ upper limit for the fractional abundance ratio of [O₂]/[H₂] is found to be X(O₂)≤1.2x10^–7 towards the Sgr A molecular belt region.