Odin observations of the Galactic centre in the 118-GHz band. Upper limit to the O2 abundance

TL;DR

This study used the Odin satellite and ground-based telescopes to search for molecular oxygen in the Galactic centre, setting upper limits on its abundance and detecting other molecular lines.

Contribution

First observational upper limits on O2 abundance in the Galactic centre using Odin satellite data, complemented by ground-based observations of related molecular lines.

Findings

No detection of O2 lines, upper limit X(O2) < 1.2 x 10^-7

Detection of HC3N emission line at 118.27 GHz

Detection of CH3OH line complex in certain clouds

Abstract

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 +20 km/s and +50 km/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 118.27 GHz, attributable to the J=13-12 HC3N line. Upper limits are presented for the 118.75-GHz O2 (1,1-1,0) ground transition line and for the 118.11-GHz 3Pi2, 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 cloud and for the 157-GHz, J=4-3, SiC line in the Sgr A +20 and +50 km/s 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 clouds but not towards Sgr A*/CND. A 3-sigma upper limit for the fractional abundance ratio of [O2]/[H2] is found to be X(O2) < 1.2 x 10exp(-7) towards the Sgr A molecular belt region.