Stellar Cycle Variability in Mount Wilson Stars and Dynamo Models: Rotation Rate and Dynamo Number Dependency

Abstract

Similar to the solar cycle, the magnetic cycles of other solar-type stars are also variable. How the variability of the stellar cycle changes with the rotation rate or the dynamo number is valuable information for understanding the stellar dynamo process. We examine the variability in stellar magnetic cycles by studying 81 stars from the data of the Mount Wilson Observatory, which started observations in 1966. For 28 stars, we have time series data available until 2003, while for others, the data are limited until 1995. We specifically explore how the variability changes with respect to three rotation-related parameters. We find a modest positive correlation between the variability and the stellar rotation period. In addition, we find suggestive negative correlations between the variability and the inverse squared Rossby number (Ro−2) and the ratio of the mean cycle duration and rotation period ( log ( 〈 Pcyc〉/ Prot)2 ). Variability computed from the magnetic field of stellar dynamo models also shows similar trends. Finally, inspired by previous studies, we examine dynamo number scaling in our model data and find that Ro−0.6 (instead of Ro−2 as suggested in the linear αΩ dynamo theory) and ( 〈 Pcyc〉 / Prot)0.6 (instead of log ( 〈 Pcyc〉/ Prot)2 as predicted in previous observations) are a good measure of the dynamo number. In conclusion, our results demonstrate that the stellar magnetic cycle variability decreases with the increase of the rotation rate or the dynamo number. © 2025. The Author(s). Published by the American Astronomical Society.