Simulations of facular magnetic fields on cool stars

I. Main-sequence stars with solar metallicity

¹ Max Planck Institute for Solar System Research, Justus-von-Liebig Weg 3, 37077 Göttingen, Germany
² Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria

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Received: 22 April 2025
Accepted: 26 December 2025

Abstract

Context. Stellar convection in the presence of magnetic field affects the emergent intensity, as well as the structure and evolution of cool main-sequence dwarfs.

Aims. We aim to understand the effect of faculae-like field strengths on near-surface stellar convection using 3D radiative MHD simulations of near-surface magneto-convection.

Methods. We compare simulations of F, G, K, and M main-sequence stars with a small-scale dynamo (SSD) to faculae-like spatially averaged field strengths (from 100 to 500 G). We focus on the effect of imposed magnetic field on the thermodynamic stratification and velocities, along with the bolometric intensity and surface field strength.

Results. Imposed magnetic fields result in reduced average density and gas pressure near the surface compared to the SSD simulations. The temperature stratification also shows a dip at and just below the stellar surface. The changes in average bolometric intensity are within a percent, with different trends with field strength for different stellar types. In addition, the convective velocities are reduced. The magnitude of changes in thermodynamic quantities are related to field strength as well as the stellar T_eff.

Conclusions. Faculae-strength magnetic fields modify the near surface convection by reducing gas pressure and density as well as suppressing convection in regions with strong field concentrations. The strength of these effects depends on the stellar type.