Previous studies have found that red giants (RGs) in close binary systems undergoing spin-orbit resonance exhibit an enhanced level of magnetic activity with respect to single RGs rotating at the same rate, from measurements of photometric variability, S'ph, and the chromospheric emission S-index, S_CaII_. 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. Secondly, we strikingly observe that the depths of the MgI and H{alpha}_ lines are anticorrelated and correlated, respectively, with the amplitude of solar-like oscillations for a given surface gravity, logg, regardless of the presence of photometric rotational modulation. Such a correlation opens up future possibilities of estimating the value of magnetic fields at the surface of RG stars, whether quiet or active, by combining spectroscopic and asteroseismic measurements with three-dimensional atmospheric models that include radiative transfer.

Cone search capability for table J/A+A/686/A93/s-lines (Chromospheric indices and associated uncertainties)