Small-scale dynamo in cool stars

Abstract

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.

Small-scale dynamo in cool main sequence stars. II. The effect of metallicity

“…So far we analysed intrinsic properties of the 3D simulations, where this study focused on different metallicities for solar effective temperature and a separate study investigated different spectral types (F3V, G2V, K0V, and M0V) (Bhatia et al 2022). However, most of the quantities calculated in 3D simulations can not be observed directly even on the Sun.…”

“…Below the optical surface, changes in pressure and density in the SSD run (Fig. 2) compared to the pure hydrodynamic simulation are rather insensitive to the metallicity and can be explained by the magnetic pressure and change of the turbulent pressure (see Bhatia et al 2022, for a more detailed discussion). Higher up in the photosphere the changes in density and pressure depend significantly on the metallicity of the star.…”

“…This is the second study in a series of papers where we overcome this omission and investigate the properties and effects of magnetic fields generated by small-scale turbulent dynamos in stars with outer convection zones. In a parallel study we investigate the SSD as a function of the stellar effective temperature (Bhatia et al 2022), where the we focused on stars of spectral types F3V, G2V, K0V and M0V with solar metallicity. In this paper we utilised the MURaM code (Vögler et al 2005) to simulate the action of SSD and small-scale fields in metal-rich (M/H=0.5) and metal-poor (M/H=-1.0) stars with solar temperature and surface gravity.…”

Can 1D radiative equilibrium models of faculae be used for calculating contamination of transmission spectra?

“…Our description of the quiet Sun is based on the most up-to-date 3D simulations as presented in Bhatia et al (2022) and Witzke et al (2022). One novel feature of these simulations is that they account for a small-scale dynamo operating near the stellar surface.…”

“…In this study we focus on a star with solar fundamental parameters and model faculae by initially adding a vertical magnetic field of 100 G, 200 G, and 300 G, which corresponds to increasingly stronger levels of magnetic activity, to the state-of-the-art MURaM simulations of small-scale dynamo (SSD) by Bhatia et al (2022) and Witzke et al (2022). In Section 2 we introduce the 3D approach for calculating facular contrasts and discuss the results.…”