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SWAS and Arecibo observations of H2O and OH in a diffuse cloud along the line-of-sight to W51

David A. Neufeld·Michael J. Kaufman·Paul F. Goldsmith·David J. Hollenbach·Rene Plume
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Abstract

Observations of W51 with the Submillimeter Wave Astronomy Satellite (SWAS) have yielded the first detection of water vapor in a diffuse molecular cloud. The water vapor lies in a foreground cloud that gives rise to an absorption feature at an LSR velocity of 6 km/s. The inferred H2O column density is 2.5E+13 cm-2. Observations with the Arecibo radio telescope of hydroxyl molecules at ten positions in W51 imply an OH column density of 8E+13 cm-2 in the same diffuse cloud. The observed H2O/OH ratio of ~ 0.3 is significantly larger than an upper limit derived previously from ultraviolet observations of the similar diffuse molecular cloud lying in front of HD 154368. The observed variation in H2O/OH likely points to the presence in one or both of these clouds of a warm (T > 400) gas component in which neutral-neutral reactions are important sources of OH and/or H2O.

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David A. Neufeld, Michael J. Kaufman, Paul F. Goldsmith, David J. Hollenbach, Rene Plume. 2002-07-16. SWAS and Arecibo observations of H2O and OH in a diffuse cloud along the line-of-sight to W51. https://doi.org/10.1086/343077

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