アイテム詳細

要旨

Context. 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, and 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 not been investigated thus far.
Aims: 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.
Methods: We studied 3D magnetohydrodynamic (MHD) models of the four stellar boxes, covering the subsurface convection zone up to the lower photosphere in a small Cartesian box, based on the MURaM radiative-MHD simulation code. We compared the SSD runs against reference hydrodynamic runs.
Results: The 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_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.
Conclusions: The 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 a reduction in the convective velocities.