The California Legacy Survey. VI. The Fate of Activity Cycles in Sun-like Stars

Abstract We analyze the spectra of 91 solar-type G- and K-type main-sequence dwarfs in the solar neighborhood to assess activity trends with stellar properties. As a follow-on work to our studies of the decoupling of rotation from age, we use chromospheric activity time series data to probe changes in stellar activity cycles as further evidence of differences in magnetic dynamos. We present evidence that the average chromospheric activity remains nearly constant while the variation in a star’s activity decreases by an order of magnitude. Measurable changes to the stellar dynamo are suggested that are not strongly correlated with the average stellar activity, but instead correlated with the amplitude of activity variations for stars beyond the age of the Sun. In contrast, while the activity cycle period is strongly correlated with effective temperature for stars aged between 2 and 4 Gyr, we find no such correlation for stars older than the Sun or otherwise low-activity stars beyond <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msubsup> <mml:mi>R</mml:mi> <mml:mi>HK</mml:mi> <mml:mo>′</mml:mo> </mml:msubsup> <mml:mo stretchy="false">)</mml:mo> </mml:math> = −4.90. By quantifying the Ca II H and K activity variation over the timescales of stellar cycles, we suggest that age dating of mature-age solar-type stars requires knowledge of activity variability in addition to the average activity.