H2O MASERS WITHOUT ASSOCIATED DIFFUSE H-II-REGIONS - AN EARLIER EVOLUTIONARY PHASE

In apparent contrast with the common belief that galactic H2O masers a re always found associated with H II regions, the majority (80%) of H2 O masers are without diffuse H II regions. The aim of this work is to confirm the lack of diffuse ionized gas around these masers, and to se e if these sources represent a homogeneous sub-class in an earlier evo lutionary phase in which a diffuse H II region has not yet formed. We present the results of a search for H89 alpha (3 cm) hydrogen recombin ation line towards a sub-sample of 60 of these masers. The detection r ate is very low: only 2 sources (3%) show recombination line emission. In order to explain this low detection rate, we have examined possibl e selection effects, such as sensitivity of the recombination line obs ervations, or lack of ionized gas because of the low luminosity of the associated star. Although both effects can play some role, by no mean s can they account for the large number of masers without associated d iffuse H II regions. The most probable answer is that masers form very early, much before the formation of a diffuse H II region, and spend most of their life in this evolutionary stage. The 20% positional agre ement between H II regions and water masers found in low resolution (a rcmin) surveys may indicate that maser emission continues for a fracti on of the main-sequence life-time of the newly formed star. Alternativ ely (and more probably), this coincidence may reflect that more than o ne star-formation event (i.e. the formation of a new maser) occurs in a stellar cluster and that these are spread over an extended period, l onger than that required by the first massive stars of the cluster to develop their own H II regions.