Simulations of facular magnetic fields on cool stars I: Main sequence stars with solar metallicity

TL;DR

This work investigates how faculae-like magnetic fields influence near-surface convection in cool main-sequence stars (F–M types) by performing 3D radiative MHD simulations using the MURaM code. Starting from small-scale dynamo–stable atmospheres, uniform vertical fields of 100–500 G are imposed to study changes in thermodynamic stratification, velocities, and emergent bolometric intensity, with 12-bin radiative transfer and 60 minutes of post-convergence data analyzed. The results show that magnetic fields evacuate plasma, reducing surface density and gas pressure, creating a dip in the temperature profile near the surface, and suppressing convective velocities, with the magnitude of these effects depending on field strength and effective temperature. Despite these changes, the bolometric intensity remains within about 1% of the SSD reference, while the morphology and distribution of magnetic fields at the optical surface vary with spectral type, influencing the interpretation of stellar spectra and exoplanet observations in magnetically active stars.

Abstract

Stellar convection in the presence of magnetic field affects the emergent intensity, as well as the structure and evolution of cool main-sequence dwarfs. 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. 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. 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 $T_{\rm eff}$. 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.

Figures (19)

• Figure 1: Snapshot of the bolometric intensity $I$ (in $10^{10}$ erg/cm$^2$/s) for the 300 G case (left column) and the corresponding vertical magnetic field $B_z$ (in kG) (middle column) and the vertical velocity $v_z$ (in km/s) (right column) for spectral types (from top to bottom) F, G, K and M, respectively. The field and velocity plots correspond to the $\tau=1$ optical surface.
• Figure 2: Same as Fig. \ref{['fig:snap_300g']}, but for the 100 G cases.
• Figure 3: Same as Fig. \ref{['fig:snap_300g']}, but for the 500 G cases.
• Figure 4: $T_{\rm eff}$ as a function of field strength, normalized by the SSD $T_{\rm eff}$. The shaded regions represent changes of $\pm 1\%$. The vertical bars at each point are $1\sigma$ average variation in $T_{\rm eff}$. The legend contains the average SSD $T_{\rm eff}$ in Kelvin for each stellar type.
• Figure 5: Density $\rho$ (top), gas pressure $p_{\rm gas}$ (middle) and temperature $T$ (bottom) for all magnetic field strengths and stellar types. The horizontal axis is the number of pressure scale heights below the surface computed for the SSD case for each star (the left side of the plot is towards the bottom boundary and the right side is towards the top). The vertical dotted line marks the height at which $\langle \tau \rangle=1$ for each case.