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Paper: Activity in A-type Stars
Volume: 479, Progress in Physics of the Sun and Stars
Page: 385
Authors: Balona, L. A.
Abstract: Kepler photometry shows that most A-type stars have low frequency variations which can be understood in terms of rotational modulation. Indeed, the distribution of equatorial velocities derived from the photometric periods agrees with the distribution of equatorial velocities of A-type stars in the general field. The amplitude of the rotational frequency varies by 20–30 percent as might be expected of star spots. From the light amplitudes we estimate that most spots are considerably larger than typical sunspots but generally smaller than the largest sunspots. The rotation peaks in the periodograms of a significant fraction of A-type stars have a peculiar structure which is not understood. Although peaks corresponding to the rotation frequency can be identified in many δ Scuti stars, the low frequency peaks in these stars are too numerous to be caused by rotational modulation. It thus appears that while the variability of non-pulsating A stars can be explained by rotation, the low-frequency variability in A-type δ Sct stars requires a new pulsation mechanism. We also find several γ Dor stars much hotter than the theoretical hot edge of the instability strip. We find 13 new A-type flare stars, which means that about 1.5 percent of A stars flare. Less dramatic flares may be common in all A-type stars. We show that these superflares cannot be attributed to normal flares on a cool companion. We conclude that A-type stars are active and, like cooler stars, have starspots and flares. Surprisingly, there does not seem to be a drop in activity as the granulation boundary is crossed.
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