Effect of surface magnetic fields on limb darkening in main-sequence stars

Stellar limb darkening encodes the thermal and radiative structure of stellar photospheres and is a key ingredient in modeling transit light curves and transmission spectra. It was recently shown that stellar surface magnetic fields modify limb darkening in stars with near-solar fundamental parameters, and that only magnetic models can reproduce high-precision transit observations for such stars. However, for stars with non-solar fundamental parameters, the magnitude of the magnetic effect on limb darkening remains unconstrained. We aim to investigate how surface magnetism affects stellar limb darkening across a range of fundamental parameters and to provide the community with center-to-limb spectra of stars at different magnetization levels. We used the MPS-ATLAS code to compute synthetic spectra from 3D radiative magnetohydrodynamic box-in-a-star simulations performed with the MURaM code. These simulations self-consistently capture photospheric magneto-convection without relying on ad hoc parameterizations. We performed calculations for main-sequence stars at solar metallicity with effective temperatures in the range T_ eff = 3200–6800 K. For stars with solar effective temperature, we also considered metal-poor, mathrm M/H = -1.0, and metal-rich, mathrm M/H = 0.5, cases. We show that the magnitude of the magnetic effect depends strongly on stellar fundamental parameters, increasing toward hotter and more metal-rich stars. Overall, limb darkening is significantly affected by magnetic fields in K-, G-, and F-dwarfs, while the effect becomes negligible in M-dwarfs. We release a public database of synthetic spectra at ten disk positions .