Empirically revealed properties of Rieger-type cycles of stellar activity

Abstract

Context. The Rieger cycles were discovered in the Sun as a specific 154-day periodicity of flare occurrence; they strongly influence terrestrial space weather. This phenomenon is far from being understood. Various proposed mechanisms for this periodicity need further verification in stars with stellar parameters different from those of the Sun. Aims. In this work, we study the Rieger-type cycle (RTC) periods PRTC of stellar activity surveyed in the photometric data of the Kepler space telescope. Methods. The processing of 1726 stellar light curves reveals statistics of PRTC values for different main-sequence stars with different effective temperatures Teff and periods of rotation P. This study uses as an index of stellar activity the squared amplitude of the first rotational harmonic A12 of the stellar light curve variability. Results. The obtained information on PRTC of the considered stars confirms the phenomenological analogy between stellar RTCs and the solar Rieger cycles. Two types of RTCs were found: (1) activity cycles with PRTC independent on the stellar rotation, which are typical for the stars with Teff ≲ 5500 K, and (2) activity cycles with PRTC proportional to the stellar rotation period P, which take place on stars with Teff ≳ 6300 K. These two types of RTCs can be driven by the Kelvin and Rossby waves, respectively. The Rossby wave-driven RTCs show a relation with the location of tachocline at shallow depths in the hot stars. This confirms the theoretical predictions of the connection of the RTC with the tachocline. At the same time, the Kelvin wave-driven RTCs do not show this connection. Apparently, both types of wave drivers of RTCs can coexist, resulting in the joint modulation of the magnetic flux tubes emergence by Kelvin and Rossby waves, and the corresponding behavior of PRTC. Conclusions. The signatures of two types of wave drivers discovered for RTCs and their different relations with the tachocline call for a revision and further elaboration of the theory of RTCs.