Astrophys. J., 539, L123-L127 (2000/August-3)
O2 in interstellar molecular clouds.
GOLDSMITH P.F., MELNICK G.J., BERGIN E.A., HOWE J.E., SNELL R.L., NEUFELD D.A., HARWIT M., ASHBY M.L.N., PATTEN B.M., KLEINER S.C., PLUME R., STAUFFER J.R., TOLLS V., WANG Z., ZHANG Y.F., ERICKSON N.R., KOCH D.G., SCHIEDER R., WINNEWISSER G. and CHIN G.
Abstract (from CDS):
We have used the Submillimeter Wave Astronomy Satellite (SWAS) to carry out deep integrations on the NJ=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)<2.6x10–7 (3 σ). A combination of data from nine sources yields <]N(O2)/N(H2)≥[0.33±1.6 (3 σ)]x10–7. 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.
ISM: Clouds - ISM: Molecules
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