Nonequilibrium Excitation of Methanol in Galactic Molecular Clouds: Multitransitional Observations at 2 Millimeters

We observed 14 methanol transitions near λ = 2 mm in Galactic star-forming regions. Broad, quasi-thermal J0-J-1E methanol lines near 157 GHz were detected toward 73 sources. Together with the 6-1-50E and 5-2-6-1E lines at 133 GHz and the 71-70E line at 166 GHz, they were used to study the methanol excitation. In the majority of the observed objects, the Class I 6-1-50E transition is inverted, and the Class II 5-2-6-1E and 60-6-1E transitions are overcooled. This is exactly as predicted by models of low-gain Class I masers. The absence of the inversion of Class II transitions 5-2-6-1E and 60-6-1E means that quasi-thermal methanol emission in all objects arises in areas without a strong radiation field, which is required for the inversion.