O2 in interstellar molecular clouds

We have used the Submillimeter Wave Astronomy Satellite (SWAS) to carry out deep integrations on the N-j = 3(3) --> 1(2) transition of O-2 in a variety of Galactic molecular clouds. We here report no convincing detection in an initial set of observations of 20 sources. We compare O-2 integrated intensities with those of (CO)-O-18 in a similarly sized beam and obtain 3 sigma upper limits for the O-2/(CO)-O-18 abundance ratio less than or equal to 2.3 in four clouds and less than or equal to 3.6 in five additional clouds. Our lowest individual limit corresponds to N(O-2)/N(H-2) < 2.6 x 10(-7) (3 sigma). A combination of data from nine sources yields < N(O-2)/N(H-2)> = [0.33 +/- 1.6 (3 sigma)] x 10(-7). These low limits, characterizing a variety of clouds in different environments at different Galactocentric radii, indicate that O-2 is not a major constituent of molecular clouds and is not an important coolant. The abundance of O-2 is significantly 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 that include the interaction of gas-phase molecules and grain surfaces and/or circulation of material between well-shielded and essentially unshielded regions. This circulation may be powered by turbulence or other driving forces that effectively keep molecular clouds chemically unevolved.