A Period-Luminosity Relation for the Slow Variations of Luminous Blue Variables

Abstract

The slow (annual-to-decadal) cycles of light and color variation displayed by many luminous blue variables show mean periods that are inversely proportional to the stars' luminosities. New theoretical evolutionary tracks reveal that very massive stars undergo repeated mass outbursts due to quasi-secular oscillations of the stellar envelope between dynamically stable and unstable states, at the observed locations of LBVs at quiescence in the H-R diagram. Within the estimated errors, the models correctly predict the observed periods, eruption mass-loss rates, and lack of dependence of these rates on the stars' luminosities. During a cycle the models shift nearly imperceptibly on the H-R diagram. Observed variations of light and color in the LBVs are therefore probably attributable to changes in size of the optically thick ejected cloud. An earlier phase of stronger dynamical instability is predicted to have led to the ejection of huge circumstellar shells, whose computed masses scale with stellar luminosity in roughly the manner observed for massive old nebulae surrounding LBVs.