Effective temperature vs. rotation period. The color bar indicates the chromospheric activity $R_{\mathrm{HK}}^{\prime }$. The green dashed lines denote the three critical Rossby number that separate the convective region, gap, the interface region, and the weakened magnetic braking region. The blue dotted line (Ro = 0.11) denotes the lower envelope of the Kepler slow rotator distribution, where fast rotators begin to converge. In order to clarify this lower envelope, we only plot the Kepler star of our sample (i.e., do not include stars of the two clusters).