Enhanced magnetic activity in rapidly rotating binary stars

Stellar activity is fundamental to stellar evolution and the formation and habitability of exoplanets. Magnetic surface activity is driven by the interaction between convective motions and rotation in cool stars, resulting in a dynamo process. In single stars, activity increases with rotation rate until it saturates for stars with rotation periods P_rot < 3–10 d. However, the mechanism responsible for saturation remains unclear. Observations indicate that red giants in binary systems that are in spin–orbit resonance exhibit stronger chromospheric activity than single stars with similar rotation rates, suggesting that tidal flows can influence surface activity. Here, we investigate the chromospheric activity of main-sequence binary stars to understand the impact of tidal forces on saturation phenomena. For binaries with 0.5 < P_rot (d) < 1, mainly contact binaries that share a common thermal envelope, we find enhanced activity rather than saturation. This result supports theoretical predictions that a large-scale α–ω dynamo during common-envelope evolution can generate strong magnetic fields. We also observe supersaturation in chromospheric activity, a phenomenon tentatively noted previously in coronal activity, where activity levels fall below saturation and decrease with shorter rotation periods. Our findings emphasize the importance of studying stellar activity in stars with extreme properties compared with the Sun’s.