Waldmeier Effect in Stellar Cycles

TL;DR

This study investigates whether the Waldmeier Effect, a key feature of solar cycles, also exists in Sun-like stars by analyzing stellar activity data, revealing that similar cycle asymmetries and correlations are present beyond the Sun.

Contribution

It provides the first evidence that the Waldmeier Effect is observable in Sun-like stars, supporting the universality of stellar dynamo mechanisms.

Findings

Most studied stars exhibit the Waldmeier Effect WE2.

WE1 is weaker and present only in some stars.

Solar and stellar cycles show similar asymmetries.

Abstract

One of the most robust features of the solar magnetic cycle is that the stronger cycles rise faster than the weaker ones. This is popularly known as the Waldmeier Effect, which is known for more than 80 years. This fundamental feature of the solar cycle has not only practical implications, e,g., in predicting the solar cycle, but also implications in understanding the solar dynamo. Here we ask the question whether the Waldmeier Effect exists in other Sun-like stars. To answer this question, we analyze the Ca \II{} H \& K S-index from Mount Wilson Observatory for 21 Sun-like G--K stars. We specifically check two aspects of Waldmeier Effect, namely, WE1: the anti-correlation between the rise times and the peaks and WE2: the positive correlation between rise rates and amplitudes. We show that except HD~16160, HD~81809, HD~155886 and HD~161239, all stars considered in the analysis show WE2. While WE1 is found to be present only in some of the stars studied. Further, the WE1 correlation is weaker than the WE2. Both WE1 and WE2 exist in the solar S-index as well. Similar to the solar cycles, the magnetic cycles of many stars are asymmetric about their maxima. The existence of the Waldmeier Effect and asymmetric cycles in Sun-like stars suggests that the dynamo mechanism which operates in the Sun is also operating in other stars.