Small-scale Dynamo in Cool Main-Sequence Stars: Effect on Stratification, Convection and Bolometric Intensity

In cool main-sequence stars, the near-surface convection has an impact on the center-to-limb variation of photospheric emission, with implications for stellar lightcurves during planetary transits. In the Sun, there is strong evidence for a small-scale dynamo (SSD) maintaining the small-scale magnetic flux. This field could affect the near-surface convection in other cool main-sequence stars.

An SSD could conceivably generate equipartition magnetic fields, which could lead to non-negligible changes not only in convection and intensity characteristics, but also in stratification. We aim to investigate these changes for F, G, K and M stars. 3D MHD models of the four stellar types covering the subsurface region to lower atmosphere in a small cartesian box are studied using the MURaM rMHD simulation code. The MHD runs are compared against a reference hydrodynamic (HD) run.

The deviations in stratification for the deeper convective layers is negligible, except for the F-star, where reduction in turbulent pressure due to magnetic fields is substantial. Convective velocities are reduced by a similar percentage for all the cases due to inhibitory effect of strong magnetic fields near the bottom boundary. All four cases show small-scale brightenings in intergranular lanes, corresponding to magnetic field concentrations, but overall effects on the r.m.s contrast and spatial powerspectra are varied.