A New Pumping Mechanism for A Series of Class II Methanol Masers in the J0 - J-1E Transitions

A new pumping mechanism – methanol masers without population inversion is presented in this paper. It can be used to explain the formation of a series of J0 → J-1E methanol masers, while the 21 → 30A+ methanol masers are regarded as a driving coherent micrwave field. In the new mechanism, the intensities of J0 - J-1E methanol masers are increased with the decreasing transition frequencies (or with rotational number J, approximately). These results agree with Slysh et al. (1995) and Slysh et al. (1999) J ≤ 5 observations for G3345.01+1.79 and W48, in which both J0 → J-1E and 21 → 30A+ methanol masers are detected coincidentally. Other astronomical conditions, such as magnetic field, 21 → 30A+ coherent radition, incoherent pumping rate by thermal radition and so on are also discussed. The new mechanism can operate as a complement to other ordinary maser pumping mechanisms for some class II methonal maser sources.