Photometric Activity Cycles in Open Clusters

Understanding the origin and evolution of magnetic activity in low-mass stars remains a central challenge in stellar astrophysics. Open clusters, with well-constrained ages, provide ideal benchmarks to trace this evolution. We compile a sample of single stars in young open clusters (<1 Gyr) to investigate how magnetic activity and its cycles depend on age and mass. To probe long-period activity cycles analogous to the 11-yr solar cycle, we develop a method that combines time-series photometry from Kepler FFIs, ASAS-SN, and ZTF. Applying this technique, we previously detected cycles in 138 fast-rotating G–K stars, revealing young Sun-like stars in the intermediate region where our Sun lies and challenging the traditional active–inactive branch paradigm (Chahal 2025). We now extend this analysis to open clusters observed in Kepler FFIs spanning 10 Myr to 1 Gyr, detecting photometric cycles in over 100 cluster members. The cycle amplitude is higher for longer cycle periods, suggesting that shorter cycles are driven by near-surface dynamos, while longer cycles likely originate from deeper convective or interface-layer dynamos. Most detected cycles occur in younger, fast-rotating stars, with a marked decline in the detection of longer cycles among older stars. We also identify several young stars lying above the traditional active branch in the cycle–rotation diagram, challenging its validity, particularly at young ages. Finally, we find a significant decrease in cycle amplitudes coincident with the spin-down stalling phase, indicating reduced dynamo efficiency, consistent with weakened internal shear from core–envelope coupling.