Methanol masers: Reliable tracers of the early stages of high-mass star formation

The GLIMPSE and MSX surveys have been used to examine the mid-infrared properties of a statistically complete sample of 6.7 GHz methanol masers. The GLIMPSE point sources associated with methanol masers are clearly distinguished from the majority, typically having extremely red mid-infrared colors similar to those expected of low-mass Class 0 young stellar objects. The intensity of the GLIMPSE sources associated with methanol masers is typically 4 mag brighter at 8.0 mu m than at 3.6 mu m. Targeted searches toward GLIMPSE point sources with [3.6] - [4.5] > 1.3 and an 8.0 mu m magnitude less than 10 will detect more than 80% of Class II methanol masers. Many of the methanol masers are associated with sources within infrared dark clouds (IRDCs), which are believed to mark regions where high-mass star formation is in its very early stages. The presence of Class II methanol masers in a significant fraction of IRDC suggests that high-mass star formation is common in these regions. Different maser species are thought to trace different evolutionary phases of the high-mass star formation process. Comparison of the properties of the GLIMPSE sources associated with Class II methanol masers and other maser species shows interesting trends, consistent with Class I methanol masers tracing a generally earlier evolutionary phase and OH masers tracing a later evolutionary phase.