Magnetic cycles at different ages of stars

TL;DR

This study analyzes magnetic activity cycles in solar-type stars over 36 years, revealing differences in cycle patterns related to stellar age and rotation, with older stars showing simpler cycles and younger stars more complex ones.

Contribution

It introduces a detailed analysis of stellar magnetic cycles using advanced time-frequency methods, highlighting the relationship between cycle complexity, rotation, and age.

Findings

Older stars have longer, smoother cycles (~9.7 years).

Younger stars exhibit shorter, more complex, and variable cycles.

A clear age division exists at around 2-3 Gyr between cycle types.

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.