Activity Cycles in Solar-Type Stars

Systematic observational studies, mainly focused on solar-type stars in the F to early K range, have detected magnetic activity cycles analogous to the solar one. When analyzing the relationship between the length of these cycles and stellar rotation rate, a non-monotonic dependence is observed. Activity periods are discretely distributed into two distinct branches: inactive and active. Intriguingly, some stars with rotation rates faster than the Sun exhibit two coexisting activity cycles, with periods simultaneously falling on both the inactive and active branches. In this work, we analyze the time series of the Mount Wilson index of a set of 44 stars, covering a range of rotation periods (sub-solar, solar, and super-solar), to identify single or double activity cycles. We detected activity cycles in 19 of the analyzed stars, with 7 of these exhibiting double-cycle behavior. In a complementary approach, we also analyze the activity cycles simulated for identical solar twins with rotation periods both shorter and longer than the Sun.