Deepak Chahal

Photometric Activity Cycles in fast-rotating Stars: Revisiting the Reality of Stellar Activity Cycle Branches

Our understanding of stellar activity cycles and their dependence on various stellar parameters is very limited, especially in the context of our Sun. In our recent study, we aim to detect magnetic 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 activity 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 non-overlapping time-series data from the Kepler Full Frame Images (FFIs), All Sky Automated Survey for SuperNovae (ASAS-SN) and Zwicky Transient Facility (ZTF), and the second set of 68 stars used data from Kepler-FFIs–ZTF. Additionally, we measured the activity cycles for 25 RS Cannum Venaticorum (RS CVn) candidates. For our sample, we also examined whether the observed flux variations correlated or anti-correlated with the photospheric activity due to the presence of faculae or starspots. We found a sample of fast-rotating K-type stars which are faculae-dominated like our Sun. 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% 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% 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 and is not in a transitional dynamo phase.