We present the results from a systematic study of temporal variation of stellar activity in young late-type stars. We used multi-epoch LAMOST (Large sky Area Multi-Object fiber Spectroscopic Telescope) low-resolution spectra of over 300 member candidates in three young open clusters: Pleiades, Praesepe, and Hyades. The spectral measurements of TiO band strength near 7050{AA} (TiO2) and equivalent width of H{alpha} line (EW_H{alpha}) are used as the tracers of cool spot coverage and chromospheric emission strength, respectively. The analysis of time-variation patterns of these two tracers suggested that there exist detectable variabilities in TiO2 and EW_H{alpha}, and their time-scales are in the wide range from days to years. Results showed that more active stars, younger and fast rotators, tend to have larger activity variations. There is a tendency of anticorrelation between temporal variations in TiO2 and EW_H{alpha}_. Also, appreciable anticorrelation in the rotational phase between H{alpha} emission and K2 brightness is detected in some M dwarfs, indicating spatial co-location of the plages with cool star-spots; however, cool stars do not always show such co-location features. Furthermore, spot coverage and H{alpha} emission were evident at all rotational phases of several M dwarfs, indicating a basal level of activity, perhaps due to many small and randomly located active regions in the atmosphere.

Cone search capability for table J/MNRAS/495/2949/table3 (Basic information of stars and their statistics of measurements)