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 2.5x10(13) cm(-2). Observations with the Arecibo radio telescope of hydroxyl molecules at 10 positions in W51 imply an OH column density of 8x10(13) cm(-2) in the same diffuse cloud. The observed H2O/OH ratio of similar to0.3 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 (Tgreater than or similar to400 K) gas component in which neutral-neutral reactions are important sources of OH and/or H2O.