Small-scale Dynamo in Cool Stars: I. Changes in stratification and near-surface convection for main-sequence spectral types

TL;DR

This study investigates how small-scale dynamo magnetic fields affect stratification and convection in the near-surface layers of cool main-sequence stars, revealing significant impacts especially in F-type stars.

Contribution

It provides the first detailed 3D MHD simulation analysis of SSD effects across multiple spectral types in cool main-sequence stars.

Findings

SSD generates magnetic fields with 5-80% of kinetic energy.

Magnetic fields cause negligible stratification changes except in F-star.

In F-star, SSD reduces convective velocities and turbulent pressure.

Abstract

Some of the small-scale solar magnetic flux can be attributed to a small-scale dynamo (SSD) operating in the near-surface convection. The SSD fields have consequences for solar granular convection, basal flux, as well as chromospheric heating. A similar SSD mechanism is expected to be active in the near-surface convection of other cool main-sequence stars, but this has never been investigated. We aim to investigate changes in stratification and convection due to inclusion of SSD fields for F3V, G2V, K0V and M0V spectral types in the near-surface convection. 3D magnetohydrodynamic (MHD) models of the four stellar boxes, covering the subsurface convection zone up to the lower photosphere in a small cartesian box, are studied using the \textit{MURaM} radiative-MHD simulation code. The SSD runs are compared against reference hydrodynamic runs. An SSD is found to efficiently produce magnetic field with energies ranging between 5\% to 80\% of the plasma kinetic energy at different depths. This ratio tends to be larger for larger $T_{\mathrm{eff}}$. The relative change in density and gas pressure stratification for the deeper convective layers due to SSD magnetic fields is negligible, except for the F-star. For the F-star, there is a substantial reduction in convective velocities due to Lorentz force feedback from magnetic fields, which, in turn, reduces the turbulent pressure. SSD in near-surface convection for cool main-sequence stars introduces small but significant changes in thermodynamic stratification (especially for the F-star) due to reduction in convective velocities.