Understanding the magnetic activity of red giants

According to dynamo theory, stars with convective envelopes efficiently generate surface magnetic fields (which manifest as magnetic activity: starspots, faculae, flares) when their rotation period is shorter than their convective turnover time. Most red giants (RG), having undergone significant spin down while expanding, have slow rotation and no spots. However, Gaulme et al. (2020) showed that out of a sample of 4500 RGs observed by the NASA Kepler space telescope, about 8% display spots. They also detected solar-like oscillations in 99.3% of the sample, and determined the evolutionary stage (hydrogen-shell or helium-core burning) of 76% of them. From complementary high-resolution spectroscopic observations of 85 targets, the active RGs can be categorized as: 1) RGs in close binary systems spun up by tidal forces; 2) solar-mass RGs that gained angular momentum by engulfing stellar or substellar companions; 3) intermediate-mass RGs that were fast rotators in the late main sequence before entering the red giant phase.