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Delta Scuti

Title: The Blazhko Effect in Delta Scuti and other Groups of Pulsating Stars
Authors: Michel Breger

Amplitude and period variations have been detected in almost all types of pulsating stars. Many of these modulations are nearly periodic. In RR Lyrae stars the phenomenon in known as the Blazhko Effect. Because of the observed similarities, we propose to extend the definition to other groups. We illustrate the Blazhko Effect with examples from Cepheids, RR Lyrae stars, White Dwarfs, sdB stars and delta Scuti variables. For delta Scuti stars the present results indicate the presence of at least two effects: beating of independent modes with close frequencies and stellar cycles. For period and amplitude changes with timescales less than one year, the beating hypothesis explains the observed modulations very well. This has been shown by the correctly correlated relationship between amplitude and phase changes as well as the repetitions of these cycles. However, the observed period variations with longer timescales cannot be due to simple beating between two close frequencies. For the star AI CVn (= 4 CVn), we can derive accurate annual frequency values for a number of radial and nonradial modes from 1974 to 2009. For prograde and retrograde modes, the frequency variations are of similar size, but with opposite signs. The radial mode shows no (or little) changes. Furthermore, all frequency variations show a reversal around 1990. These results are consistent with long-term cycles affecting individual modes differently with some common systematic behaviour. The observed change in the size of rotational splitting is interpreted as a change in the differential rotation during the stellar cycle.

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