Water masers associated with compact molecular clouds and ultracompact Hii regions: The extended sample

Abstract

We present the results of a survey of water maser emission at 22.2 GHz towards a selected sample of IRAS-PSC sources which are believed to be associated with very young massive stars. The sample consists of 591 sources. The observations have been carried out using the Medicina 32-m radiotelescope, operated by the Istituto di Radioastronomia - C.N.R., Bologna. Whereas previous searches for maser emission have been directed towards known H II regions, the aim of the present survey is to identify new sources in an even earlier evolutionary phase, corresponding to the development of an ultracompact H II region. It is shown that our sample contains a significant number of sources that could be considered good candidates of massive protostellar cores still in the main accretion phase. (Author (revised))