Water Abundance in Molecular Cloud Cores

We present Submillimeter Wave Astronomy Satellite (SWAS) observations of the 1₁₀ → 1₀₁ transition of ortho-H₂O at 557 GHz toward 12 molecular cloud cores. The water emission was detected in NGC 7538, ρ Oph A, NGC 2024, CRL 2591, W3, W3OH, Mon R2, and W33 and was not detected in TMC-1, L134N, and B335. We also present a small map of the H₂O emission in S140. Observations of the H₂¹⁸O line were obtained toward S140 and NGC 7538, but no emission was detected. The abundance of ortho-H₂O relative to H₂ in the giant molecular cloud cores was found to vary between 6 × 10^-10 and 1 × 10^-8. Five of the cloud cores in our sample have previous H₂O detections; however, in all cases the emission is thought to arise from hot cores with small angular extents. The H₂O abundance estimated for the hot core gas is at least 100 times larger than in the gas probed by SWAS. The most stringent upper limit on the ortho-H₂O abundance in dark clouds is provided in TMC-1, where the 3 σ upper limit on the ortho-H₂O fractional abundance is 7 × 10^-8.