Searching for O2 in the SMC: Constraints on oxygen chemistry at low metallicities

Christine D. Wilson (1) , Henrik Olofsson (2) , Laurent Pagani (3) , R. S. Booth (2) , Urban Frisk (4) , Åke Hjalmarson (2) , Michael Olberg (2) , Aage Sandqvist (5)

1 Department of Physics and Astronomy, McMaster University
2 OSO - Onsala Space Observatory, Chalmers University of Technology
3 LERMA - Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique
4 SSC - Swedish Space Corporation
5 Stockholm Observatory, AlbaNova University Center

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^-1 in a 9' (160 pc) diameter beam corresponds to an upper limit on the O2/H2 abundance ratio of < 1.3 × 10^-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.

Domaines: Astrophysique [astro-ph]

Christine D. Wilson, Henrik Olofsson, Laurent Pagani, R. S. Booth, Urban Frisk, et al.. Searching for O2 in the SMC: Constraints on oxygen chemistry at low metallicities. Astronomy and Astrophysics - A&A, 2005, 433, pp.L5-L8. ⟨hal-03732219⟩