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

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.

arXiv comment4 pages, accepted to A&A Letters

References in corpus (6)

• A Quantitative Comparison of SMC, LMC, and Milky Way UV to NIR Extinction Curves
• The Metallicity Dependence of the CO-to-H${}_2$ Conversion Factor from Observations of Local Group Galaxies
• Ten eclipsing binaries in the Small Magellanic Cloud: fundamental parameters and SMC distance
• First NH3 detection of the Orion Bar
• The Abundance and Emission of H2O and O2 in Clumpy Molecular Clouds
• Evolution of the Abundance of CO, O2, and Dust in the Early Universe

Cited by in corpus (5)

• A PDR-Code Comparison Study
• Herschel Measurements of Molecular Oxygen in Orion
• Molecular line intensities as measures of cloud masses - II. Conversion factors for specific galaxy types
• Odin observations of the Galactic centre in the 118-GHz band. Upper limit to the O2 abundance
• Astrophysics in 2005