Discovery of new 19.9-GHz methanol masers in star-forming regions

We have used the NASA Tidbinbilla 70-m antenna to search for emission from the 2(1)-3(0) E (19.9-GHz) transition of methanol. The search was targeted towards 22 star formation regions that exhibit maser emission in the 107.0-GHz 3(1)-4(0) A(+) methanol transition, and in the 6.6-GHz 5(1)-6(0) A(+) transition characteristic of class II methanol maser sources. A total of seven sources were detected in the 2(1)-3(0) E transition, six of these being new detections. Many of the new detections are weak (less than or similar to0.5 Jy), however, they appear to be weak masers rather than thermal or quasi-thermal emission. We find a strong correlation between sources that exhibit 19.9-GHz methanol masers and those that both have the class II methanol masers projected against radio continuum emission and have associated 6035-MHz OH masers. This suggests that the 19.9-GHz methanol masers arise in very specific physical conditions, probably associated with a particular evolutionary phase. In the model of Cragg, Sobolev & Godfrey these observations are consistent with gas temperatures of 50 K, dust temperatures of 150-200 K and gas densities of 10(6.5)-10(7.5) cm(-3).