Nonequilibrium excitation of methanol in galactic molecular clouds: Multitransitional observations at 2 millimeters

We observed 14 methanol transitions near lambda = 2 mm in Galactic star-for ming regions. Broad, quasi-thermal J(0)-J(-1)E methanol lines near 157 GHz were detected toward 73 sources. Together with the 6(-1)-5(0) E and 5-(2)-6 (-1) E lines at 133 GHz and the 7(1)-7(0) E line at 166 GHz, they were used to study the methanol excitation. In the majority of the observed objects, the Class I 6-(1)-5(0) E transition is inverted, and the Class II 5-(2)-6- (1) E and 6(0)-6-(1) E transitions are overcooled. This is exactly as predi cted by models of low-gain Class I masers. The absence of the inversion of Class II transitions 5-(2)-6-(1) E and 6(0)-6-(1) E means that quasi-therma l methanol emission in all objects arises in areas without a strong radiati on field, which is required for the inversion.