Maser action without inversion in interstellar space

Abstract

It is recognized that interstellar 6.7 and 12.2 GHz methanol masers toward Class II sources are associated with each other and coexist toward ultracompact HII regions. Therefore we suggest a new excitation mechanism – methanol masers without population inversion. This mechanism can explain the formation of 6.7 GHz methanol masers while the 12.2 GHz methanol masers are regarded as driving coherent microwave fields. It is discussed that this mechanism is associated with astronomical conditions.