Magnetic cycles at different ages of stars

Abstract

We study the different patterns of interannual magnetic variability in stars on or near the lower main sequence, approximately solar-type (G-K dwarf) stars in time series of 36 years from the Mount Wilson Observatory Ca\,{\sc ii}\,H\&K survey. Our main aim is to search for correlations between cycles, activity measures and ages. Time-frequency analysis has been used to discern and reveal patterns and morphology of stellar activity cycles, including multiple and changing cycles, in the datasets. Both the results from short-term Fourier transform and its refinement using the Choi-Williams distribution, with better frequency resolution, are presented in this study. Rotational periods of the stars were derived using multi-frequency Fourier analysis.From the studied 29 stars we found at least one activity cycle on 28 of them. Twelve stars, with longer rotational periods ($39.7\pm6.0$ days) have simple, smooth cycles, and the rest of the stars, with on-average much faster rotation ($18.1\pm12.2$ days) show complex and sometimes vigorously changing, multiple cycles. The cycles are longer and quite uniform in the first group ($9.7\pm1.9$ years), while are generally shorter and with greater variety in the second one ($7.6\pm4.9$). There is a clear age division between stars with smooth and complex cycles that follows the known separation between the older and younger stars at around 2 to 3~Gyr of age.