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 O2 towards the region of strongest molecular emission in the Small Magellanic Cloud. Our 3σ upper limit to the O2 integrated intensity of < 0.049 K km s −1 in a 9′ (160 pc) diameter beam corresponds to an upper limit on the O2/H2 abundance ratio of < 1.3 × 10 . 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 subsolar 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.