Abstract It is recognized that the interstellar methanol-107 GHz masers and OH-4.765 GHz masers towards Class II sources are associated with each other and coexist towards ultracompact HII regions. Therefore we suggest a new pumping mechanism -- methanol masers without population inversion. It can explain the formation of 107 GHz methanol masers, with the 4.765 GHz OH masers acting as a driving coherent microwave field. It is argued that this mechanism is compatible with the astronomical conditions.
Keywords H II regions --- masers --- radiation mechanisms: non-thermal --- ISM: molecules --- line: formation
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