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 $S'_{ph}$ and chromospheric emission S-index $S_{Ca_{II}}$. Here, we consider a sample of 4465 RGs observed by the NASA Kepler mission 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$\alpha$, 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 illustrated by an NUV excess and shallower H$\alpha$ 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 red giants -- single or binary --, based on their rotation periods and magnetic activity indices. Secondly, we strikingly observe that the depths of the Mg$_{I}$ and H$\alpha$ lines are anti-correlated 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 including radiative transfer.