The first high-resolution observations of 19.9-ghz methanol masers

We have used the Australia Telescope Compact Array to undertake the first high-resolution observations of the 19.9-GHz (2₁-3₀E) methanol maser transition. The emission is coincident with the location of the targeted 6.7-GHz methanol masers to within 0.2 arcsec in absolute position. We find that the relative distribution of the 19.9-GHz maser emission in our sample differs from that observed in the 6.7-GHz transition, in contrast to the similar distribution often seen in the 6.7-and 12.2-GHz methanol masers in many sources. We also find that the peak velocity for the 19.9-GHz methanol masers is frequently different from that observed in other class II transitions in the same regions. These two results suggest that while 19.9-GHz methanol masers arise from the same general location as other class II transitions, they are likely not coincident on milliarcsecond scales, which has implications for multitransitional modelling of class II methanol maser sources. We have investigated the properties of the OH and other class II methanol transitions for those 107-GHz methanol masers with and without an associated 19.9-GHz methanol maser. On the basis of these comparisons, we suggest that the 19.9-GHz transition traces an evolutionary phase around the peak of the class II maser luminosity into its decline.