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P.F. Goldsmith (NAIC, Cornell U.), M.L.N. Ashby, E.A. Bergin, S.C. Kleiner, G.J. Melnick, B.M. Patten, R. Plume, J.R. Stauffer, V. Tolls, Z. Wang, Y.F. Zhang (CfA), M. Harwit (Cornell U.), N.R. Erickson, J.E. Howe, R.L. Snell (UMass, Amherst), D.A. Neufeld (JHU), D.G. Koch (NASA ARC), R. Schieder, G. Winnewisser (U. Köln), G. Chin (NASA GSFC)
We have used SWAS to carry out deep integrations on the N\rm J = 33arrow12 transition of O2~ in a variety of Galactic molecular clouds. We here report no convincing detection in an initial set of observations of 20 sources. We compare O2~ integrated intensities with those of C18O~ in a similar sized beam, and obtain 3 \sigma upper limits to the O2/C18O~ abundance ratio \leq 2.5 in four clouds, and \leq 5 in six additional clouds. Our lowest individual limit corresponds to N(O2)/N(H2) < 2.6\times10-7. These low limits, characterizing a variety of clouds in different environments at different Galactocentric radii, indicate that O2~ is not a major constituent of molecular clouds and is not a significant coolant. The abundance of O2~ is appreciably lower than predicted by steady--state single--component chemical models. The present results are best understood in the context of cloud chemical and dynamical models incorporating circulation of material between well--shielded and essentially unshielded regions. This circulation may be powered by turbulence or other driving forces which effectively keep molecular clouds chemically relatively unevolved. We acknowledge support from NASA contract NAS5-30702 and from the National Science Foundation.