The Red MSX Source survey: Ammonia and water maser analysis of massive star-forming regions

The Red MSX Source (RMS) survey has identified a sample of ?1200 massive young stellar objects (MYSOs), compact and ultra-compact H II regions from a sample of ?2000 MSX and Two Micron All Sky Survey (2MASS) colour-selected sources. We have used the 100-m Green Bank Telescope to search for 22–24 GHz water maser and ammonia (1,1), (2,2) and (3,3) emission towards ?600 RMS sources located within the northern Galactic plane. We have identified 308 H2O masers which corresponds to an overall detection rate of ?50 per cent. We find no significant difference in the detection rate for H II regions and MYSOs which would suggest that the conditions required to produce maser emission are equally likely in both phases. Comparing the detection rates as a function of luminosity, we find the H2O detection rate has a positive dependence on the source luminosity, with the detection rate increasing with increasing luminosity.

We detect ammonia emission towards 479 of these massive young stars, which corresponds to ?80 per cent. Ammonia is an excellent probe of high-density gas allowing us to measure key parameters such as gas temperatures, opacities and column densities, as well as providing an insight into the gas kinematics. The average kinetic temperature, full width at half-maximum linewidth and total NH3 column density for the sample are approximately 22 K, 2 km s?1 and 2 × 1015 cm?2, respectively. We find that the NH3 (1,1) linewidth and kinetic temperature are correlated with luminosity, and finding no underlying dependence of these parameters on the evolutionary phase of the embedded sources, we conclude that the observed trends in the derived parameters are more likely to be due to the energy output of the central source and/or the linewidth–clump mass relationship.

The velocities of the peak H2O masers and the NH3 emission are in excellent agreement with each other, which would strongly suggest an association between the dense gas and the maser emission. Moreover, we find the bolometric luminosity of the embedded source and the isotropic luminosity of the H2O maser are also correlated. We conclude from the correlations of the cloud and water maser velocities and the bolometric and maser luminosity that there is a strong dynamical relationship between the embedded young massive star and the H2O maser.