SIMBAD references

By using the LAMOST time-domain survey data, we study stellar activities based on the H_α lines for about 2000 stars in four K2 plates. Two indices, ${R}_{{\rm{H}}\alpha }^{{\prime} }$ and ${R}_{{\rm{H}}\alpha }^{+}$, are computed from LAMOST spectra, the former of which is derived by excluding the photospheric contributions to the H_α lines, while the latter is derived by further subtracting the non-dynamo-driven chromospheric emission. Meanwhile, the periodicity and variation amplitudes are computed from K2 light curves. Both the ${R}_{{\rm{H}}\alpha }^{{\prime} }$-Ro relation and ${R}_{{\rm{H}}\alpha }^{+}$-Ro relation show complicated profiles in the nonsaturated decay region. Hot stars show flatter slopes and a higher activity level than cool stars, and the behavior is more notable in the ${R}_{{\rm{H}}\alpha }^{+}$-R_o relation. This is consistent with recent studies using other activity proxies, including L_x/L_bol, ${R}_{\mathrm{HK}}^{{\prime} }$, and amplitudes of optical light curves. This may suggest different kinds of stars follow different power laws in the decay region. Most of our targets have multiple observations, and some of them exhibit significant variability of Hα emissions, which may cause the large scatters shown in the decay region. We find three targets exhibiting positive correlation in a rotational phase, possibly indicating that their optical light curves are dominated by hot faculae rather than cool starspots.