Magnetic activity of red giants: Correlation between the amplitude of solar-like oscillations and chromospheric indicators

Abstract

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, Sph′, and the chromospheric emission S-index, SCa II. Here, we consider a sample of 4465 RGs observed by the NASA Kepler mission, for which previous studies have measured Sph′ and SCa II, 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 Hα, Mg I, and infared Ca II absorption lines from LAMOST spectroscopic data. Firstly, as for Ca II 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 Hα and infrared Ca II lines. We report no excess of Mg I 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 Mg I and Hα lines are anticorrelated and correlated, respectively, with the amplitude of solar-like oscillations for a given surface gravity, log g, 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.