Multiple short-term cycles in Kepler star KIC 6876668

¹ Georg-August-Universität, Institut für Astrophysik Friedrich-Hund-Platz 1 37077 Göttingen, Germany
² E. Kharadze Georgian National Astrophysical Observatory Mount Kanobili, Georgia
³ School of Natural Sciences and Medicine, Ilia State University Cholokashvili Ave. 3/5 Tbilisi, Georgia
⁴ Institute of Physics, IGAM, University of Graz Universitätsplatz 5 8010 Graz, Austria
⁵ INAF-Osservatorio Astrofisico di Catania, Via S. Sofia 78 95123 Catania, Italy
⁶ Departament de Física & Institut d’Aplicacions Computacionals de Codi Comunitari (IAC3), Universitat de les Illes Balears 07122 Palma de Mallorca, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 19 July 2025
Accepted: 21 November 2025

Abstract

Context. Solar and stellar magnetic activity may have similar properties, which can be used to understand the stellar magnetic evolution.

Aims. This paper examines the multiple short-term cycles in the total irradiance of the Sun and in the light curve of a Kepler star, KIC 6876668.

Methods. We study the light curve variations of KIC 6876668, which exhibits a rotation period of 5−6 d. In order to detect cyclic patterns across rotational and Rieger timescales, we used wavelet and Lomb-Scargle (LS) methodologies.

Results. We found that the light curve of the star exhibits periodicities at ∼47, 59 and 72 d, which are fully consistent with the Rieger period range. Through LS and wavelet analysis, we also found variations in the total irradiance of the Sun as a star in cycle 23 of ∼185, 240, and 380 d. The ratios of stellar and solar cyclic periods over their rotation periods revealed striking similarities of ∼8, ∼10, and 12−15. The observed cycles are interpreted as the spherical harmonics of magnetic Rossby waves in dynamo layers of the star and the Sun. The corresponding magnetic field strengths are estimated as 40 kG and 10 kG in the stellar and solar interiors, respectively. The relationship observed between the Rieger cycle and the rotation period of a young star suggests that the processes in its inner layers driving stellar cycles resemble those found in the Sun.

Conclusions. These results provide an insight into the internal magnetic processes in a Sun-like star, KIC 6876668, where a short cycle and a strong magnetic field indicate stronger activity. Sun-like stars as well as other stars may show similar variations in magnetic activity to the Sun. Therefore, the cycles can be used to extract plasma parameters in their interior. Our analysis shows that the ratio of the rotation angular frequency and the estimated magnetic field strength might stay constant (Ω/B = constant) throughout stellar evolution.