Interstellar masers as evolutionary tracers of high-mass star formation

Determining an evolutionary clock for high-mass star formation is a much sought after result in astrophysics, as it will allow comprehensive quantitative studies of the formation of high-mass stars to be completed. We have carried out detailed studies of a large number of sources suspected of undergoing highmass star formation and have found that common masers transitions can be very helpful in determining their relative evolutionary stage. We present new observations of 22 GHz water masers and 12.2 GHz methanol masers, completed with the Australia Telescope Compact Array and the Parkes radio telescope, respectively. Hundreds of maser sources are reported here for the first time. From the large samples, we are able to confidently report detection statistics, as well as characteristics of the source populations. Analysis of the maser velocities, flux densities, luminosities and variability are presented. We have investigated the relative evolutionary phases of high-mass star formation associated with the presence or absence of combinations of the 22 GHz water and 12.2 GHz methanol masers that we detect, along with the locations of 6.7 GHz methanol and main-line OH masers. The locations of the different maser species have been compared with the positions of 1.2 mm dust clumps, molecular data, radio continuum, GLIMPSE point sources and Extended Green Objects. Comparison between the characteristics of coincident sources has revealed strong evidence for an evolutionary sequence for the different maser species in high-mass star formation regions. We present our proposed sequence for the presence of the common maser species associated with young high-mass stars, along with the data supporting our arguments. Insights into the characteristics and physical conditions associated with the different relative evolutionary stages are presented. We find a general trend whereby both the water and methanol masers increase in luminosity as they evolve. Further to this, we find evidence that the methanol and water masers increase in velocity range as they evolve, implying that the gas volume conducive to the maser emission also increases with evolution. The rate of increase of the 12.2 GHz methanol maser luminosity and velocity range is lower than that of their 6.7 GHz counterparts. We find evidence to suggest that water maser sources also increase in luminosity less rapidly than 6.7 GHz methanol maser sources. We additionally find evidence to suggest that the increase in luminosity and velocity range of water maser sources 'turns over' towards the later stages of evolution, presumably decreasing until the maser emission ceases. We find no evidence for the methanol masers to show a similar decrease in either luminosity or velocity range, implying that they cease emitting much more abruptly. Comparisons with mid-infrared data show that the masers themselves are more sensitive to evolutionary changes than the associated mid-infrared emission.