Photometric activity cycles in fast-rotating stars: Revisiting the reality of stellar activity cycle branches

TL;DR

This study analyzes photometric data over 14 years to investigate activity cycles in fast-rotating young stars, challenging the traditional classification of stellar activity cycle branches and suggesting the Sun's position is not unique.

Contribution

It provides new observational evidence that the clear separation of activity cycle branches may not be as distinct as previously believed, especially for G-K-type stars.

Findings

Fast-rotating G-K stars show no strong cycle length-rotation period correlation.

34% of stars fall in the intermediate region between known activity branches.

23% of young Sun-like stars are in the intermediate region, similar to the Sun.

Abstract

We aim to detect activity cycles in young main-sequence stars, analogous to the 11-year solar cycle, using combined photometric survey data. This research will enhance our understanding of how cycle periods relate to rotation rates in fast-rotating stars. We measured activity cycles for 138 G-K-type main sequence stars using combined time-series photometry spanning ~14 years. The first set of 70 stars used data from Kepler Full Frame Images (FFIs)-ASAS-SN-ZTF, and the second set of 68 stars used data from Kepler-FFIs-ZTF. Additionally, we measured the activity cycles for 25 RS CVn candidates. For our sample, we analyzed the correlation or anti-correlation between flux variations and photospheric activity, which arises due to the presence of faculae or starspots. We identified fast-rotating K-type stars that are faculae-dominated by tracking spot or faculae evolution in Kepler RMS data. Our findings reveal that fast-rotating G-K-type stars show no strong correlation between cycle length and rotation period. Previous studies have identified active and inactive branches in the cycle-rotation diagram. However, we find that G-K-type stars do not show a clear trend aligning with the active branch, with 34 per cent of stars falling within the intermediate region between the two branches, where our Sun resides. Our results highlight that the proposed distinction between the two branches may not be as definitive as previously thought, particularly regarding the active branch. Furthermore, we also detected 23 per cent of young Sun-like stars in the intermediate region, where our Sun is located, implying that our Sun may not be unique in this regard.