Chromospheric activity in 4 open clusters

We measured excess equivalent widths of H{alpha}, H{beta}, and CaII K based on estimated chromospheric contributions from old and inactive field dwarfs, and excess luminosities are obtained by normalizing bolometric luminosity, for more than 700 late-type stars in these open clusters.

Results indicate two activity sequences in cool spot coverage and H{alpha} excess emission among GK dwarfs in Pleiades and M dwarfs in Praesepe and Hyades, paralleling with well-known rotation sequences. A weak dependence of chromospheric emission on rotation exists among ultrafast rotators in saturated regime with Rossby number Ro<=0.1. In the unsaturated regime, chromospheric and coronal emission show similar dependence on Ro, but with a shift towards larger Ro, indicating chromospheric emission gets easily saturated than coronal emission, and/or convective turnover time-scales based on X-ray data do not work well with chromospheric emission.