Generation of Interstellar Class II 72-81A + and 72-81A − Methanol Masers ∗

Abstract

New methanol maser lines at 72?63A?(86.6 GHz) and 72?63A+(86.9 GHz) together with two candidate methanol maser lines at 72?81A?(80.99 GHz) and 72?81A+(111.29 GHz) have been detected in W3(OH). We use a pumping mechanism, i.e., methanol masers without population inversion, to explain the formation of weak methanol masers of 72?81A+ and 72?81A?. We explain well why the line-shape of the transition 72?81A+ is not typical. A similar argument can be applied to the ?-type level system 72A?, 63A? and 81A?, as well as to the 72?81A? 80.99 GHz masers.