Water maser emission from comets

We used the Very Large Array in 1997 January, February, March, and April to observe comet Hale-Bopp in the 1.35 cm H2O line. No line was detected in a ny of the four observations, and upper limits to the H2O line emission are presented. These upper limits are typically around 60 mJy, which is much lo wer than the similar to 2.5 Jy Bur density reported for this line in comet Bradfield (1974b) and jansky-level flux densities sporadically reported for a few other comets. This discrepancy is most likely due to a combination o f questionable reliability of past reported detections and the possibility that masing in particular observational geometries is required for detectio n of the line. We concur with previous studies that indicated that this lin e could be detected only if it were masing. In the only masing scenario lik ely to yield a positive result, a line may be seen only if the masing regio n is observed along a jet against the radio continuum background of the nuc leus. We estimate the probabilities of a detection in a random search for t wo typical positions of the origin of a jet on a rotating nucleus: (1) clos e to the pole and (2) close to the equator. Because the probabilities are v ery low (much less than 0.01) in either case, our negative result for comet Hale-Bopp las well as previous negative results for other comets) cannot e xclude the possibility of such masing jets. Carefully scheduled observation s of future comets whose rotation and jet locations have been determined mu st be used to evaluate this possibility.