Letter to the Editor
Searching for O2 in the SMC:*
Constraints on oxygen chemistry at low metallicities
Department of Physics & Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada e-mail: email@example.com
2 Onsala Space Observatory, 439 92 Onsala, Sweden
3 LERMA & UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
4 Swedish Space Corporation, PO Box 4207, 171 04 Solna, Sweden
5 Stockholm Observatory, SCFAB-AlbaNova, 106 91 Stockholm, Sweden
Accepted: 12 February 2005
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 < K km s-1 in a 9′ (160 pc) diameter beam corresponds to an upper limit on the O2/H2 abundance ratio of <. 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.
Key words: galaxies: individual: SMC / ISM: molecules / astrochemistry
Based on observations with Odin, a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB), the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes), and Centre National d'Etudes Spatiales (CNES). The Swedish Space Corporation was the industrial prime contractor and is also responsible for the satellite operation.
© ESO, 2005