Searching for O$_2$ in the SMC:Constraints on Oxygen Chemistry at Low Metallicities

TL;DR

This study used the Odin satellite to set an upper limit on molecular oxygen in the Small Magellanic Cloud, providing insights into oxygen chemistry at low metallicities and the effects of photo-dissociation and dust grain freeze-out.

Contribution

First observational constraint on O2 abundance in the SMC at low metallicity, informing astrochemical models of oxygen chemistry in such environments.

Findings

O2/H2 abundance ratio < 1.3E-6 in the SMC

Low O2 abundance likely due to photo-dissociation effects

Dust grain freeze-out could also explain the low O2 levels

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 3sigma upper limit to the O2 integrated intensity of <0.049 K km/s in a 9'(160 pc) diameter beam corresponds to an upper limit on the O2/H2 abundance ratio of <1.3E-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.