Masers as signposts of high-mass protostars

Abstract

The 22 GHz H2O maser line was observed towards 79 candidate high-mass protostellar objects from a flux-limited sample of 6.7 GHz methanol sources. The emission was detected in 41 sources, towards 28 of these for the first time. The detection rate of 52% was similar to rates reported for other samples of high-mass protostars selected mainly with far-infrared (FIR) colour criteria. The median value of H2O maser luminosity of 10 −5.5 Lis equal to that of the CH3OH maser luminosity, whereas the median OH maser luminosity was found to be ∼1.5 orders of magnitude lower. Comparison of the velocity ranges showing maser emission implies that for the majority of sources the H2 Oa nd CH 3OH maser lines originate from different regions. The percentage of sources with emission in two or three of the maser species, their association with radio contin- uum and IR emission and the maser and IR luminosities are consistent with the view that evolutionary phases with H2 Oa nd CH3OH masers largely overlap and precede the OH maser phase, while at a later stage OH and CH3OH masers may coexist. Strong correlations of OH and CH3OH maser luminosities with IR luminosity and only a marginal correlation of H2 Oa nd IR luminosity confirm current pumping schemes of all three maser lines.