2002ApJ...580..278N

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 2.5x10¹³ cm^–2. Observations with the Arecibo radio telescope of hydroxyl molecules at 10 positions in W51 imply an OH column density of 8x10¹³ 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≳400K) gas component in which neutral-neutral reactions are important sources of OH and/or H₂O.