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Issue A&A
Volume 433, Number 1, April I 2005
Page(s) L5 - L8
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200500024



A&A 433, L5-L8 (2005)
DOI: 10.1051/0004-6361:200500024

Letter

Searching for O $\mathsf{_2}$ in the SMC:

Constraints on oxygen chemistry at low metallicities
C. D. Wilson1, A. O. H. Olofsson2, L. Pagani3, R. S. Booth2, U. Frisk4, Å. Hjalmarson2, M. Olberg2 and Aa. Sandqvist5

1  Department of Physics & Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada
    e-mail: wilson@physics.mcmaster.ca
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

(Received 20 December 2004/ Accepted 12 February 2005 )

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 3$\sigma$ upper limit to the O2 integrated intensity of <0.049 K km s-1 in a 9$^\prime$ (160 pc) diameter beam corresponds to an upper limit on the O2/H2 abundance ratio of < $ 1.3 \times 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.


Key words: galaxies: individual: SMC -- ISM: molecules -- astrochemistry

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© ESO 2005


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