Multiple and changing cycles of active stars II. Results

TL;DR

This study analyzes decades-long observations of 20 active stars, revealing that most exhibit multiple, variable activity cycles that are likely driven by a common dynamo mechanism, with a confirmed relation between rotation and cycle periods.

Contribution

It provides the first comprehensive analysis of multiple and changing stellar activity cycles using time-frequency methods on long-term data.

Findings

15 stars show definite multiple cycles

Cycle periods are systematically variable

A positive relation between rotation and cycle periods is confirmed

Abstract

We study the time variations of the cycles of 20 active stars based on decades-long photometric or spectroscopic observations. A method of time-frequency analysis, as discussed in a companion paper, is applied to the data. Fifteen stars definitely show multiple cycles; the records of the rest are too short to verify a timescale for a second cycle. The cycles typically show systematic changes. For three stars, we found two cycles in each of them that are not harmonics, and which vary in parallel, indicating that a common physical mechanism arising from a dynamo construct. The positive relation between the rotational and cycle periods is confirmed for the inhomogeneous set of active stars. Stellar activity cycles are generally multiple and variable.