Statistical properties of 12.2GHz methanol masers associated with a complete sample of 6.7GHz methanol masers

We present definitive detection statistics for 12.2GHz methanol masers toward a complete sample of 6.7GHz methanol masers detected in the Methanol Multibeam survey south of declination -20°. In total, we detect 250 12.2GHz methanol masers toward 580 6.7GHz methanol masers. This equates to a detection rate of 43.1%, which is lower than that of previous significant searches of comparable sensitivity. Both the velocity ranges and the flux densities of the target 6.7GHz sources surpass that of their 12.2GHz companion in almost all cases. Eighty percent of the detected 12.2GHz methanol maser peaks are coincident in velocity with the 6.7GHz maser peak. Our data support an evolutionary scenario whereby the 12.2GHz sources are associated with a somewhat later evolutionary stage than the 6.7GHz sources devoid of this transition. Furthermore, we find that the 6.7GHz and 12.2GHz methanol sources increase in luminosity as they evolve. In addition to this, evidence for an increase in velocity range with evolution is presented. This implies that it is not only the luminosity but also the volume of gas conducive to the different maser transitions that increases as the sources evolve. Comparison with GLIMPSE mid-infrared sources has revealed a coincidence rate between the locations of the 6.7GHz methanol masers and GLIMPSE point sources similar to that achieved in previous studies. Overall, the properties of the GLIMPSE sources with and without 12.2GHz counterparts are similar. There is a higher 12.2GHz detection rate toward those 6.7GHz methanol masers that are coincident with extended green objects. © 2011. The American Astronomical Society. All rights reserved.