Magnetic activity variability of nearby bright Sun-like stars by 4-year intensive H$\alpha$ line monitoring

Abstract

We report intensive monitoring of the activity variability in the H$\alpha$ line for 10 Sun-like stars using the 1.88-m reflector at Okayama Branch Office, Subaru Telescope, during the last four years 2019-2022. Our aim was to investigate features of the stellar magnetic activity behaviors. We correlated the H$\alpha$ line variability of each star with the stellar activity levels derived from the Ca II H&K line, suggesting its efficiency as a magnetic activity indicator. In analyzing the H$\alpha$ line variation, we observed that some stars exhibited linear or quadratic trends during the observation period. Among several G- and K-type stars expected to have co-existing activity cycles, we confirmed the 2.9-yr short cycle of $\epsilon$ Eri (K2V) from the H$\alpha$ observations. Additionally, we established upper limits on the H$\alpha$ variability of $\beta$ Com (G0V) and $\kappa$$^1$ Cet (G5V) concerning their expected shorter cycles. We also detected the possibility of short-term activity cycles in two F-type stars, $\beta$ Vir (F9V; $\sim$ 530 days) and $\alpha$ CMi (F5IV-V; $\sim$ 130 days). The cycle in $\alpha$ CMi was observed in only one season of our 4-yr observations, suggesting the temporal absence of the cycle period. However, for stars with planets, we did not observe significant magnetic activity variability likely associated with the planetary orbital period. It is speculated that the impact of H$\alpha$ variability on radial velocity (RV) measurements may vary with spectral type.