WATER MASERS ASSOCIATED WITH DENSE MOLECULAR CLOUDS AND ULTRACOMPACT H-II REGIONS

We present the results of a survey aimed at studying the occurrence of the H2O 22 GHz maser emission from bright IR sources in star forming regions. The survey was carried out using the Medicina 32-m radiotelescope. A sample of 260 sources was selected from the IRAS Point Source Catalogue, following the colour selection criteria suggested by Richards et al. (1987) to identify dense molecular clouds where star formation might be at the very earliest stages. There are 125 sources out of the 260 that satisfy the requirements for being associated with ultracompact H II regions, according to the criteria defined by Wood & Churchwell (1989). We detected water vapor emission towards 32 of these 125 sources (27 are new detections), with a detection rate of 26%. In the remaining 135 objects, 12 (9%) new maser sources were found, all characterized by rather low integrated fluxes and narrow components. On the basis of the FIR colour indices and the maser detection rates, and from a comparison with other classes of objects, we suggest that the sources included in the subsample not associated with ultracompact H II regions contain a high fraction of candidate molecular clouds with embedded high mass protostars still in the accretion phase.