Signature of the spin-down stalling in the stellar magnetic activity

Once main-sequence (MS) solar-like stars transitioned from the saturated to the unsaturated regime in magnetic activity, their evolution was perceived as monotonic, with rotation rate and activity gradually decaying. However, Kepler revolutionized this perspective, showing that the unsaturated regime splits into different regimes. One transition, causing a stalling in the MS spin down, is attributed to core-envelope coupling and the consequent angular momentum transfer between a fast core and a slow envelope. In this work, we find evidence of this transition in the magnetic activity of the full Kepler MS sample and NGC 6811. We investigate the magnetic activity evolution in the Kepler MS sample by computing activity sequences according to the relative rotation. We find the sequences clustering at two distinct regions: 1) at high activity levels coinciding with stars near the transition above, where a behavior inversion is observed; and 2) at low activity levels corresponding to slow rotators close to the detection limit potentially facing a weakening of the magnetic braking. Focusing on NGC 6811, we find evidence for enhanced magnetic activity, that remains for significant timescales, in stars that have already transitioned. These results support the recent proposition that the strong shear encountered during the core-envelope coupling phase can cause enhanced activity. This study will help shed light on the interplay between rotation, magnetic activity, and their evolution.