O[TINF]2[/TINF] 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 C 18 O in a similarly sized beam and obtain 3 σ upper limits for the O 2 /C 18 O abundance ratio ≤ 2.3 in four clouds and ≤ 3.6 in five additional clouds. Our lowest individual limit corresponds to N (O 2 )/ N (H 2 ) < 2.6 × 10 -7 (3 σ). A combination of data from nine sources yields ⟨ N (O 2 )/ N (H 2 )⟩ = [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 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.