Starspots, lithium, and radius inflation: stellar magnetism as an astrophysical toolkit

Starspots and magnetic fields blanket the surfaces of cool stars, leading to discrepancies between precision observations of active stars and their model-derived radii, masses, and ages. Such discrepancies complicate our interpretations of these systems. Magnetic stellar evolution models including the inhibition of convection due to flux-blocking starspot complexes or strong surface magnetic fields appear plausibly more consistent with observations. Until recently, however, a lack of interpretable stellar activity data has prevented us from definitively testing this scenario. Our technique, which recovers spectroscopic starspot filling fractions from high-resolution near-infrared spectra in APOGEE, has produced a catalog of hundreds of thousands of precision starspot measurements (LEOPARD), allowing us to identify mean stellar activity relations and identify departures from it. We apply our starspot analysis to two classic problems in stellar evolution, including 1) the perceived "lithium spread", a problem where the observed lithium abundances of young stars at fixed mass varies in a way fundamentally inexplicable by standard models, and 2) the observed "radius inflation" of stars, where precision radii measurements of cool, active stars are systematically larger than model predictions. We discuss how our precision starspot data clarifies current theoretical and observational efforts in stellar physics, and how its consideration can help enhance our understanding of exoplanetary and galactic astrophysics.