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

ABSTRACT The chromospheric activity of low-mass stars in open clusters is crucial for investigating activity–age–rotation relations. We present the chromospheric activity indicator ($R^{\prime }_{\rm H\alpha }$) 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 $T_{\rm eff}$, and rotation. The activity–$T_{\rm eff}$ 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 $T_{\rm eff}$. In contrast, the activity of K-type stars tends to flatten or even weaken as $T_{\rm eff}$ decreases. We establish an empirical log$R^{\prime }_{\rm H\alpha }$–$T_{\rm eff}$–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 $T_{\rm eff}$, 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 $T_{\rm eff}$–$P_{\rm rot}$ diagram. These results provide new observational constraints on the evolution of angular momentum in stars of different spectral types.