RGs Chromospheric indices from LAMOST spect.

Here, we consider a sample of 4465 RGs observed by the NASA Kepler mission, for which previous studies have measured S'_ph_ and S_CaII_, in order to measure additional activity indicators that probe different heights in the chromosphere: the near-ultraviolet (NUV) excess from NASA GALEX photometric data, and chromospheric indices based on the depth of H_{alpha}_, MgI, and infrared CaII absorption lines from LAMOST spectroscopic data.

Firstly, as for CaII H&K, we observe that RGs belonging to close binaries in a state of spin-orbit resonance display larger chromospheric emission than the cohort of RGs, as is illustrated by an NUV excess and shallower H_{alpha}_ and infrared CaII lines. We report no excess of MgI emission. This result reinforces previous claims that tidal locking leads to enhanced magnetic fields, and allows us to provide criteria to classify active RGs - single or binary - based on their rotation periods and magnetic activity indices.