2014MNRAS.441.3137Q

Data for a number of OH maser lines have been collected from surveys. The positions are compared with recent mid-infrared (MIR) surveys such as the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) and Wide-field Infrared Survey Explorer (WISE) surveys, restricting the comparison to point sources. The colours and intensities of the IR sources are compared. There are many 18-cm OH masers, but far fewer in lines arising from higher energy levels. We also make a comparison with 5-cm Class II methanol masers. We have divided the results into three subsamples: those associated with OH masers only, those associated with OH masers and Class II methanol masers and those associated with Class II methanol masers only. There are no obvious differences in the colour-colour or colour-magnitude results for the GLIMPSE point sources. However, according to the results from the WISE 22-µm survey, the sources associated with OH masers are brighter than those associated with methanol masers. We interpret the presence of OH and methanol masers as marking the locations of regions where stars are forming. The OH masers are located on the borders of sharp features found in the IR. These are referred to as bubbles. If the OH masers mark the positions of protostars, the result provides indirect evidence for triggered star formation caused by the expansion of the bubbles.