2004MNRAS.354..401E

We have used the NASA Tidbinbilla 70-m antenna to search for emission from the 2₁-3₀E (19.9-GHz) transition of methanol. The search was targeted towards 22 star formation regions that exhibit maser emission in the 107.0-GHz 3₁-4₀A⁺ methanol transition, and in the 6.6-GHz 5₁-6₀A⁺ transition characteristic of class II methanol maser sources. A total of seven sources were detected in the 2₁-3₀E 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/cm3.