Astronomy and Astrophysics – Astrophysics
Scientific paper
2002-07-16
Astrophys.J. 580 (2002) 278-284
Astronomy and Astrophysics
Astrophysics
15 pages (AASTeX) including 4 (eps) figures. To appear in the Astrophysical Journal
Scientific paper
10.1086/343077
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.
Goldsmith Paul F.
Hollenbach David J.
Kaufman Michael J.
Neufeld David A.
Plume Rene
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