SIMBAD references

In apparent contrast with the common belief that galactic H₂O masers are always found associated with HII regions, the majority (80%) of H₂O masers are without diffuse HII regions. The aim of this work is to confirm the lack of diffuse ionized gas around these masers, and to see if these sources represent a homogeneous sub-class in an earlier evolutionary phase in which a diffuse HII region has not yet formed. We present the results of a search for H89α (3 cm) hydrogen recombination line towards a sub-sample of 60 of these masers. The detection rate is very low: only 2 sources (3%) show recombination line emission. In order to explain this low detection rate, we have examined possible selection effects, such as sensitivity of the recombination line observations, or lack of ionized gas because of the low luminosity of the associated star. Although both effects can play some role, by no means can they account for the large number of masers without associated diffuse HII regions. The most probable answer is that masers form very early, much before the formation of a diffuse HII region, and spend most of their life in this evolutionary stage. The 20% positional agreement between HII regions and water masers found in low resolution (arcmin) surveys may indicate that maser emission continues for a fraction of the main-sequence life-time of the newly formed star. Alternatively (and more probably), this coincidence may reflect that more than one 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, longer than that required by the first massive stars of the cluster to develop their own HII regions.