SENSITIVE LIMITS ON THE WATER ABUNDANCE IN COLD LOW-MASS MOLECULAR CORES

摘要

We present Submillimeter Wave Astronomy Satellite observations of water vapor in two cold starless clouds, B68 and core D in ρ Ophiuchi. Sensitive nondetections of the 1_(10)―1_(10) transition of o-H_2O are reported for each source. Both molecular cores have been previously examined by detailed observations that have characterized the physical structure. Using these rather well-defined physical properties and a Monte Carlo radiation transfer model, we have removed one of the largest uncertainties from the abundance calculation and set the lowest water abundance limit to date in cold low-mass molecular cores. These limits are x(o-H_2O) < 3 x 10~(-8) (relative to H_2) and x(o-H_2O) < 8 x 10~(-9) in B68 and ρ Oph D, respectively. Such low abundances confirm the general lack of ortho-water vapor in cold (T< 20 K) cores. Provided that the ortho/para ratio of water is not near zero, these limits are well below theoretical predictions and appear to support the suggestion that most of the water in dense low-mass cores is frozen onto the surfaces of cold dust grains.