2005A&A...433L...5W

We present a 39h 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 <0.049K.km/s in a 9' (160pc) diameter beam corresponds to an upper limit on the O₂/H₂ abundance ratio of <1.3x10^–6. 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.