$R_{\mathrm{H}\alpha }^{\mathrm{\prime }}$ versus the Rossby number R_O. The symbols are color coded by T_eff. Upper limits are highlighted with downward gray arrows. The magenta dashed line denotes the relation of Newton et al. (2017), while the cyan full line (slope = −0.18 ± 0.02) is our best fit to the cold stars (T_eff ≤ 4500 K) in the saturated activity domain (R_O < 0.21). The power law with slope −1.9 ± 0.2 we found for the stars with R_O ≥ 0.21 is shown with a green line. The 1σ errors are indicated by the hatched regions.