THE OCCURRENCE OF H2O MASERS IN THE EARLY STAGES OF STAR-FORMATION

The results of a survey of 22.2 GHz water maser emission toward 39 you ng stellar objects (McCutcheon et al. 1991) are presented. The sample contains bright IRAS sources associated with molecular clumps, which m ay represent different phases of the early evolution of star forming r egions, depending on the presence or absence of compact H II regions. We associate 19 water masers with the IRAS sources, 4 of which are new detections. Taking into account also previous detections of masers th at were quiescent during our survey, the total number of sources which show maser emission is 22 (56% of the whole sample). This has to be t aken as a lower limit if variability of the maser emission is consider ed. The maser percentage does not change for sources with or without a compact H II region. It is higher (68%) if we consider only sources a ssociated with large CO linewidths. Our H2O observations confirm that water masers can occur at the earliest phases of the formation of high luminosity stars, much before the development of an ionized region de tectable in the radio continuum, and that they are closely connected w ith molecular outflows.