Chromospheric activity $R_{\mathrm{HK}}^{\prime }$ as a function of B−V in our sample. The color bar indicates the rotation period. The Sun is marked as ⊙ symbol with B−V = 0.64, $\log R_{\mathrm{HK}}^{\prime }=-4.94$ (Egeland et al. 2017). The gray dashed line denotes S_MWO = 0.15, below which S_MWO were obtained by extrapolation that could have large uncertainties. The typical error of $\log R_{\mathrm{HK}}^{\prime }=-4.5$ is plotted as a black circle at the bottom right. The top and right histograms show the number distribution along $R_{\mathrm{HK}}^{\prime }$ and B−V, respectively.