Submillimeter Wave Astronomy Satellite and Arecibo Observations of H2O and OH in a Diffuse Cloud along the Line of Sight to W51

Observations of W51 with the Submillimeter Wave Astronomy Satellite 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-1. The inferred water column density is 2.5 × 1013 cm-2. Observations with the Arecibo radio telescope of hydroxyl molecules at 10 positions in W51 imply an OH column density of 8 × 1013 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 K) gas component in which neutral-neutral reactions are important sources of OH and/or H2O.