Magnetic activity of red giants: impact of tidal interactions on magnetic fields

According to dynamo theory, stars with convective envelopes efficiently generate surface magnetic fields, which manifest as magnetic activity in the form of starspots, faculae, flares, when their rotation period is shorter than their convective turnover time. Most red giants, having undergone significant spin down while expanding, have slow rotation, thus no spots. However, some red giants that belong to close binary systems display a photospheric activity measured from photometric time series produced by the NASA Kepler mission, which is about an order of magnitude larger than that of single red giants with similar rotation periods. In order to investigate whether binarity leads to larger magnetic fields when tides lock systems, or if a different spot distribution on single versus close binary red giants can explain this fact, we measured the chromospheric emission in the CaII H & K lines of LAMOST optical spectra. We find that red giants belonging to binary systems in a configuration of spin-orbit resonance display significantly larger chromospheric emission than single stars, suggesting that tidal interactions lead to larger magnetic fields. In other words, the large magnetic field of red giants in close binary systems is not only due to the faster rotation rate induced by tidal interactions. Somehow, our work resuscitates an old speculation about a special binary-induced dynamo activity.