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 methanol maser transition. The emission is coincident with the location of the targeted 6.7-GHz methanol masers to within 0.2 arcseconds 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 multi-transitional modelling of class II methanol maser sources. We have investigated the properties of the OH and other classII 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.