O₂ in Interstellar Molecular Clouds

We have used the Submillimeter Wave Astronomy Satellite (SWAS) to carry out deep integrations on the N_J = 3₃ → 1₂ 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 similarly sized beam and obtain 3 σ upper limits for the O₂/C¹⁸O abundance ratio ≤ 2.3 in four clouds and ≤ 3.6 in five additional clouds. Our lowest individual limit corresponds to N(O₂)/N(H₂) < 2.6 × 10^-7 (3 σ). A combination of data from nine sources yields ⟨N(O₂)/N(H₂)⟩ = [0.33 ± 1.6 (3 σ)] × 10^-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 an important coolant. The abundance of O₂ 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.