WATER IN DENSE MOLECULAR CLOUDS

We have used the G.P. Kuiper Airborne Observatory (KAO) to make initial observations of the half-millimeter ground-state transition of water in seven giant molecular clouds and in two late-type stars. No significant detections (> 3-sigma) were made, and the resulting upper limits are significantly below those expected from other, indirect observations and from several theoretical models. The implied interstellar H2O/CO abundance is less than 0.003 (3-sigma) in the cores of three giant molecular clouds. This value is less than expected from cloud chemistry models and also than estimates based on HDO and H3O+ observations. The limits are derived on the basis of a radiative transfer model incorporating LVG analysis for the low-opacity cloud exterior. Possible explanations for the low implied interstellar H2O abundance are (1) that fragmentation in dense clouds may cause unexpectedly large levels of ionizing UV radiation, (2) that water may be effectively frozen onto grains, or (3) that the cloud exterior effectively diffuses the emergent radiation.