Collision-collision pumping of cosmic masers

A mechanism which relies solely on collisional transitions is proposed to explain the pumping of cosmic masers. The conditions required for the feasibility of collision-collision pumping, which involves interactions of the masing molecules with two types of incident particles at different temperatures, are examined, and an expression is derived for the pumping rate. It is then shown that the necessary conditions of gas density, degree of ionization and incident particle temperature for collision-collision pumping may exist in known cosmic masers, and advantages of the mechanism over previous ones, which cannot satisfactorily account for the most powerful maser sources, are summarized.