We use the LAMOST spectra of member stars in Pleiades, M34, Praesepe, and Hyades to study how chromospheric activity varies as a function of mass and rotation at different age. 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. More interestingly, our analysis shows fully convective slow rotators obey the rotation-chromospheric activity relation similar to hotter stars, confirming the previous finding. We found correlations among H{alpha}, H{beta}, and CaII K emissions, in which H{alpha} losses are more important than CaII K for cooler and more active stars. In addition, a weak correlation is seen between chromospheric emission and photospheric activity that shows dependence on stellar spectral type and activity level, which provides some clues on how spot configuration varies as a function of mass and activity level.

Cone search capability for table J/MNRAS/476/908/tablef1 (Measurements for stars in Pleiades)

Cone search capability for table J/MNRAS/476/908/tablef2 (Measurements for stars in M34)

Cone search capability for table J/MNRAS/476/908/tablef3 (Measurements for stars in Praesepe)

Cone search capability for table J/MNRAS/476/908/tablef4 (Measurements for stars in Hyades)