Chromospheric activity of FGKM-type dwarfs in open clusters up to 6 Gyr

The chromospheric activity of low-mass stars in open clusters is crucial for investigating activity–age–rotation relations. We present the chromospheric activity indicator (RHα) of 8493 FGKM-type stars in 152 open clusters with ages ranging from 20 Myr to 6 Gyr, and investigate the relations among chromospheric activity, age, effective temperature Teff, and rotation. The activity–Teff relations reveal different chromospheric activity trends in single and binary stars of different spectral types. In late F-, G-, and early M-type single and binary stars, activity increases rapidly with a decrease in Teff. In contrast, the activity of K-type stars tends to flatten or even weaken as Teff decreases. We establish an empirical logRHα–Teff–age relation that enables the estimation of age. Compared to gyrochronology in open clusters, this relation can persist to 6 Gyr. Meanwhile, the chromospheric activity of K-type stars experiencing stalled spin-down increases with increasing Teff, and the rate of increase in activity increases with age. This implies a connection between the stalled spin-down of K-type stars and their increasing chromospheric activity. Moreover, the activity–rotation relations exhibit four regions: supersaturation, weak decay, rapid decay, and flat, which correspond to distinct rotation sequences in the Teff–Pᵣₒₜ diagram. These results provide new observational constraints on the evolution of angular momentum in stars of different spectral types.