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

Abstract

We have used the NASA Tidbinbilla 70-m antenna to search for emission from the 21‐30 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 31‐40 A + methanol transition, and in the 6.6-GHz 51‐60 A + transition characteristic of class II methanol maser sources. A total of seven sources were detected in the 21‐30 E transition, six of these being new detections. Many of the new detections are weak (0.5 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 . Ke yw ords: masers ‐ stars: formation ‐ ISM: molecules ‐ radio lines: ISM.