OH and H₂O Maser Activities Among Long-period Variables

The distribution within an infrared color–magnitude diagram of variable red giants known to have circumstellar masers is reported. Stars with H 2 O-maser sources occur within a color and period domain of ( J − K s ) > 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 − K s ) > 1.35 and P > 316 days, and can coexist with H 2 O-maser sources. The results suggest that red giants must evolve to absolute magnitudes brighter than M K s = − 6.8 prior to enabling H 2 O-maser emission, and possibly expand beyond a certain size before triggering OH masers.