Flare-like activity of red giant stars

The present investigation is concerned with the evidence for flare-like activity among higher luminosity cool stars, taking into account red giants and supergiants. In a Mg II study, Mullan and Stencel (1982) have observed several cool giants and supergiants repeatedly. The observations showed a variation in total emission strength, or strength/velocity of circumstellar Mg II absorption. A radio flare on the red giant Alpha Ceti was reported by Boice et al. (1981). Goldberg et al. (1982) have reported the measurement of polarization changes in the red supergiant Alpha Ori which were correlated with changes in the brightness distribution of the extended chromosphere revealed in narrow band (H-alpha) speckle interferometry. The available limited synoptic data suggest that among red giants and supergiants, flare-like events occur on timescales in the range from 10,000 to 10 million seconds.