Searching for O2 in the SMC: Constraints on Oxygen Chemistry at Low Metallicities

We present a 39 h integration with the Odin satellite on the ground-state 118.75 GHz line of O 2 towards the region of strongest molecular emission in the Small Magellanic Cloud. Our 3σ upper limit to the O 2 integrated intensity of <0.049 K km s -1 in a 9' (160 pc) diameter beam corresponds to an upper limit on the O 2 /H 2 abundance ratio of <1.3 x 10 -6 . Although a factor of 20 above the best limit on the O 2 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 O 2 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.