Stellar Chromospheric Activities Revealed from the LAMOST-K2 Time-domain Survey

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 H α 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

Stellar Chromospheric Activity and Age Relation from Open Clusters in the LAMOST Survey

We identify member stars of more than 90 open clusters in the LAMOST survey. With the method of Fang et al.(2018), the chromospheric activity (CA) indices logR'CaK for 1091 member stars in 82 open clusters and logR'H{\alpha} for 1118 member stars in 83 open clusters are calculated. The relations between the average logR'CaK, logR'H{\alpha} in each open cluster and its age are investigated in different Teff and [Fe/H] ranges. We find that CA starts to decrease slowly from logt = 6.70 to logt = 8.50, and then decreases rapidly until logt = 9.53. The trend becomes clearer for cooler stars. The quadratic functions between logR' and logt with 4000K < Teff < 5500K are constructed, which can be used to roughly estimate ages of field stars with accuracy about 40% for logR'CaK and 60% for logR'H{\alpha}