Letter to the Editor

Searching for O₂ in the SMC:^*

Constraints on oxygen chemistry at low metallicities

¹ Department of Physics & Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada e-mail: wilson@physics.mcmaster.ca
² Onsala Space Observatory, 439 92 Onsala, Sweden
³ LERMA & UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
⁴ Swedish Space Corporation, PO Box 4207, 171 04 Solna, Sweden
⁵ Stockholm Observatory, SCFAB-AlbaNova, 106 91 Stockholm, Sweden

Received: 20 December 2004
Accepted: 12 February 2005

Abstract

We present a 39 h integration with the Odin satellite on the ground-state 118.75 GHz line of O₂ towards the region of strongest molecular emission in the Small Magellanic Cloud. Our 3σ upper limit to the O₂ integrated intensity of < K km s^-1 in a 9′ (160 pc) diameter beam corresponds to an upper limit on the O₂/H₂ abundance ratio of <. Although a factor of 20 above the best limit on the O₂ abundance obtained for a Galactic source, our result has interesting implications for understanding oxygen chemistry at sub-solar metal abundances. We compare our abundance limit to a variety of astrochemical models and find that, at low metallicities, the low O₂ abundance is most likely produced by the effects of photo-dissociation on molecular cloud structure. Freeze-out of molecules onto dust grains may also be consistent with the observed abundance limit, although such models have not yet been run at sub-solar initial metallicities.