2020ApJ...900..173V

Long-term photometric variability can sometimes be related to the emergence of starspots in the photosphere following a quasi-periodic magnetic activity cycle. In order to investigate the existence and properties of magnetic cycles in a narrow range of fundamental parameters, we focused on solar-type stars in the intermediate-age open cluster NGC 6811 (1 Gyr). The cluster membership was analyzed both in terms of stellar kinematics and Gaia photometry. Rotation periods and photometric variability were analyzed for 138 stars from Kepler light curves and we discuss the existing trends and relationships. We also searched a sample of 11 solar-type stars for cycle-like periodicities and in each case classified the variability as either cyclic, multicyclic, flat, or acyclic. The results suggest a significant scatter, possibly due to sensitive nonlinearities in the dynamo process. This raises theoretical questions on whether these periodicities can be associated to activity cycles and what determines the stellar cycle presence or its period. Also, the lack of solar-twin flat activity stars in NGC 6811 combined with other recent results, suggests that the presence of magnetic grand minima in the Sun is a relatively recent phenomenon.