Fractional chromospheric Ca II H and K flux normalized to bolometric flux on a logarithmic scale, log , as a function of absolute magnitude . For each star, log is calculated with different T_eff calibrations, each plotted with a different color. Vertical gray lines connect different measurements of the same star. Triangles indicate stars exhibiting Hα emission. Filled symbols indicate stars without known planetary systems, and open symbols indicate stars with known planetary systems.