OH and H2O Maser Activities Among Long-period Variables

Abstract

The distribution within an infrared color–magnitude diagram of variable red giants known to have circumstellar masers is reported. Stars with H2O-maser sources occur within a color and period domain of (J − Ks) > 1.15 and P > 199 days, respectively, often in the absence of OH masers. By contrast, OH masers are mostly detected among cooler red giants with (J − Ks) > 1.35 and P > 316 days, and can coexist with H2O-maser sources. The results suggest that red giants must evolve to absolute magnitudes brighter than prior to enabling H2O-maser emission, and possibly expand beyond a certain size before triggering OH masers.