Magnetic activity of red giants: a near-UV and H$α$ view, and the enhancing role of tidal interactions

C. Gehan, D. Godoy-Rivera, P. Gaulme

Published 2024-01-24Version 1

Previous studies have found that red giants in close binary systems undergoing spin-orbit resonance exhibit an enhanced level of magnetic activity from measurements of the indices of photometric variability $S_{ph}$ and chromospheric emission $S_{CaII}$. Here, we complement the previous works by measuring two other indicators of chromospheric activity: the near-ultraviolet (NUV) excess $\Delta m_{NUV}$ using GALEX data, as well as the H$\alpha$ chromospheric index $S_{H\alpha}$ using spectroscopic data from LAMOST. We consider a sample of 4465 single and binary red giants observed by Kepler, and measure $\Delta m_{NUV}$ and $S_{H\alpha}$ for 842 and 3362 targets, respectively. We investigate the correlations between $\Delta m_{NUV}$, $S_{H\alpha}$, $S_{ph}$ and $S_{CaII}$, which probe magnetic activity at different heights from the photosphere to the upper chromosphere. We also find that red giants exhibiting low-amplitude oscillations tend to exhibit larger $\Delta m_{NUV}$ values, but no larger $S_{H\alpha}$ values. Importantly, we show that red giants in a close-binary configuration with spin-orbit resonance or tidal locking display significantly larger $\Delta m_{NUV}$ values and $S_{H\alpha}$ values than single red giants or red giants in binary systems with no special tidal configuration. This result reinforces previous claims that tidal locking leads to larger magnetic fields. We provide criteria to classify the active red giants (single or binary), based on their rotation period and activity indices. Since $\sim$ 90 millions stars have UV photometric observations from GALEX, and $\sim$ 10 million stars have LAMOST spectra, where the signal-to-noise ratio is higher in the vicinity of the H$\alpha$ line than the CaII H & K lines, the NUV excess and the H$\alpha$ index represent highly valuable activity indicators that could help identifying tidally-interacting stellar systems.