Stellar Surface Magnetic Fields Impact Limb Darkening

TL;DR

This paper demonstrates that small-scale magnetic fields on stellar surfaces significantly influence limb darkening, explaining discrepancies in observed exoplanet transit light curves and enabling magnetic field measurements via limb darkening analysis.

Contribution

The study introduces new 3D magneto-hydrodynamic stellar atmosphere models that incorporate magnetic fields to accurately reproduce observed limb darkening.

Findings

Magnetic fields cause less steep brightness drop towards the limb.

Models with magnetic fields match Kepler, TESS, JWST observations.

Limb darkening can be used to measure stellar magnetic fields.

Abstract

Stars appear darker at their limbs than at their disk centers because at the limb we are viewing the higher and cooler layers of stellar photospheres. Limb darkening derived from state-of-the-art stellar atmosphere models systematically fails to reproduce recent transiting exoplanet light curves from the Kepler, TESS, and JWST telescopes -- stellar brightness obtained from measurements drops less steeply towards the limb than predicted by models. All previous models assumed atmosphere devoid of magnetic fields. Here we use our new stellar atmosphere models computed with the 3D radiative magneto-hydrodynamic code MURaM to show that small-scale concentration of magnetic fields on the stellar surface affect limb darkening at a level that allows us to explain the observations. Our findings provide a way forward to improve the determination of exoplanet radii and especially the transmission spectroscopy analysis for transiting planets, which relies on a very accurate description of stellar limb darkening from the visible through the infrared. Furthermore, our findings imply that limb darkening allows measuring the small-scale magnetic field on stars with transiting planets.