Infrared Pumping Processes for SiO Masers

Abstract

A pumping mechanism for SiO masers by the infrared continuum radiation is proposed, where the population inversion is due to the anisotropic trapping of radiation in the expanding gas with a large velocity gradient. The numerical calculations show that the masers of the transitions J = 1-0 and 2-1 in the first and the second vibrationally excited states of SiO occur in the envelope of infrared stars. The observed radial velocities of the maser emissions and the acceleration mechanism of the gas are discussed.