H2O MASERS ASSOCIATED WITH DENSE MOLECULAR CLOUDS AND ULTRACOMPACT HII-REGIONS .2. THE EXTENDED SAMPLE

We present water maser observations towards a sample of candidate comp act molecular cores selected on the basis of their far-infrared colour s according to the criteria specified in a previous paper (Palla et al . 1991). We have extended our initial sample to objects with flux at 6 0 mum below 100 Jy. Our results confirm the existence of a sharp cutof f in H2O detections for log10(F25/F12) < 0.57. We demonstrate that sou rces above this threshold are likely to be associated with ultracompac t HII regions, whereas the rest of the sample is probably made of high mass (proto)stars with only a minor contamination of young HII region s.