Water abundance in molecular cloud cores

We present Submillimeter Wave Astronomy Satellite (SWAS) observations of th e 1(10)-->1(01) transition of ortho-H2O at 557 GHz toward 12 molecular clou d cores. The water emission was detected in NGC 7538, rho Oph A, NGC 2024, CRL 2591, W3, W3OH, Mon R2, and W33 and was not detected in TMC-1, L134N, a nd B335. We also present a small map of the H2O emission in S140. Observati ons of the (H2O)-O-18 line were obtained toward S140 and NGC 7538, but no e mission was detected. The abundance of ortho-H2O relative to H-2 in the gia nt molecular cloud cores was found to vary between 6 x 10(-10) and 1 x 10(- 8). Five of the cloud cores in our sample have previous H2O detections; how ever, in all cases the emission is thought to arise from hot cores with sma ll angular extents. The H2O abundance estimated for the hot core gas is at least 100 times larger than in the gas probed by SI VAS. The most stringent upper limit on the ortho-H2O abundance in dark clouds is provided in TMC-I , where the 3 sigma upper limit on the ortho-H2O fractional abundance is 7 x 10(-8).