Water Abundance in Molecular Cloud Cores

Abstract

We present Submillimeter Wave Astronomy Satellite (SWAS) observations of the 1_{10}-1_{01} transition of ortho-water at 557 GHz toward 12 molecular cloud cores. The water emission was detected in NGC 7538, Rho Oph A, NGC 2024, CRL 2591, W3, W3(OH), Mon R2, and W33, and was not detected in TMC-1, L134N, and B335. We also present a small map of the water emission in S140. Observations of the H_2^{18}O line were obtained toward S140 and NGC 7538, but no emission was detected. The abundance of ortho-water relative to H_2 in the giant molecular cloud cores was found to vary between 6x10^{-10} and 1x10^{-8}. Five of the cloud cores in our sample have previous water detections; however, in all cases the emission is thought to arise from hot cores with small angular extents. The water 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-water abundance in dark clouds is provided in TMC-1, where the 3-sigma upper limit on the ortho-water fractional abundance is 7x10^{-8}.Comment: 5 pages, 3 Postscript figures, uses aastex.cls, emulateapj5.sty (included), and apjfonts.sty (included

Observations of Absorption by Water Vapor toward Sagittarius B2

“…This is in acceptable agreement with the value of in-A ∼ 25 V dicated by the extinction map of Schultheis et al (1999 6 # 10 that significantly exceeds the abundances derived from SWAS emission-line observations of dense cloud cores (Snell et al 2000). The gas probed by our absorption-line observations is typically less dense than that studied by means of emissionline observations, so the discrepancy may reflect a real chemical effect.…”

“…Hütte-meister et al (1993Hütte-meister et al ( , 1995 have argued previously for the presence of such an envelope based on observations of ammonia lines toward Sgr B2. This explanation for the absence of strong H 2 16 O emission from Sgr B2 is of general interest since it raises the possibility that diffuse envelopes similarly scatter the emergent H 2 16 O radiation in other sources (Snell et al 2000).…”

“…The gas probed by our absorption-line observations is typically less dense than that studied by means of emissionline observations, so the discrepancy may reflect a real chemical effect. On the other hand, the emission-line observations could substantially underestimate the true water abundance if significant foreground absorption is diminishing the observed line flux densities (Snell et al 2000), a possibility that is raised by the km s Ϫ1 observations discussed below. 3.3 # 10 munication; see also Neufeld et al 1997).…”

Abundant gas-phase H2O in absorption toward massive protostars

“…In addition to the high inferred gas-phase H 2 O abundances from the infrared absorption bands, SWAS observations of sources in our sample indicate much lower H 2 O abundances of ∼10 −9 to a few ×10 −8 in the outer region (Snell et al 2000). This suggests that a jump in the H 2 O abundance is present for our sources and that the ISO-SWS observations trace primarily the warmer gas in the inner envelope.…”

“…These are in agreement with a variety of chemical models, including high-T chemistry and shocks, which predict enhanced H 2 O abundances for T > ∼ 230-300 K where most of the oxygen is driven into H 2 O, as well as grain-mantle evaporation predicting enhanced H 2 O abundances for T > ∼ 100 K (Charnley 1997;Doty et al 2002). On the other hand, observations of the H 2 O 1 10 −1 01 line at 557 GHz obtained in a large (∼4 ) beam with the Submillimeter Wave Astronomy Satellite (SWAS) show much lower gasphase abundances of ∼10 −9 to a few 10 −8 in the cold (T < ∼ 50 K) gas (Snell et al 2000;Ashby et al 2000).…”