Water masers as an early tracer of star formation

We present a study of correlation between 22 GHz water maser emission and far infrared/submillimeter (IR/sub-mm) sources. The generalized linear model (GLM) is used to predict H2O maser detection in a particular source with defined physical parameters. We checked the GLM predictions by observing a sample of selected sources with the Effelsberg 100-m telescope. In total, 359 sources were observed. The H2O masers were detected in 123 sources, with 59 new detections. We found 22 sources with a significant flux variability. Using the GLM analysis, we estimate that 2392±339 star formation regions (SFRs) in the Galaxy may harbour H2O masers detectable for single-dish observations at the noise level of ∼ 0.05 Jy. Analyzing the luminosity to mass ratio (L/M) of the ATLASGAL and Hi-GAL clumps associated with different maser species, we find that 22 GHz water masers have significantly lower values of L/M in comparison to 6.7 GHz class II methanol and 1665 MHz OH masers. This implies that 22 GHz water masers may appear prior to 6.7 GHz methanol and OH masers in the evolutionary sequence of SFRs. From the analysis of physical offsets between host clumps and maser interferometric positions, we found no significant difference between H2O and class II methanol maser offsets against host clump position. We conclude that the tight association between water masers and IR/sub-mm sources may provide insight into the pumping conditions of these masers and evolutionary stages of their onset.