Two-Temperature Pumping of H 2O Masers

Detailed calculations, including optical depth effects, are presented for the pumping of H2O masers by collisions with neutrals and electrons held at two different temperatures. The proposal by Strelnitskij (1980) that such pumping can result in arbitrarily high brightness temperatures is corroborated, but only when the electrons are cooler than the neutrals. When the neutrals are cooler than the electrons, the inversion is less efficient than what would be obtained by single-temperature pumping with any of the individual species alone.