Small-scale magnetic fields on late-type M-dwarfs

Abstract

We performed kinematic studies of the evolution of small-scale magnetic fields in the surface layers of M-dwarfs. We solved the induction equation for a prescribed velocity field, magnetic Reynolds number ReM, and boundary conditions in a Cartesian box, representing a volume comprising the optically thin stellar atmosphere and the uppermost part of the optically thick convective envelope. The velocity field is spatially and temporally variable, and stems from detailed radiationhydrodynamics simulations of convective flows in a proto-typical late-type M-dwarf (Teff = 2800 K, log g = 5.0, solar chemical composition, spectral type ≈M6). We find dynamo action for ReM ≥ 400. Growth time scales of the magnetic field are comparable to the convective turn-over time scale (≈150 sec). The convective velocity field concentrates the magnetic field in sheets and tubular structures in the inter-granular downflows. Scaling from solar conditions suggests that field strengths as high as 20 kG might be reached locally. Perhaps surprisingly, ReM is of order unity in the surface layers of cooler M-dwarfs, rendering the dynamo inoperative. In all studied cases we find a rather low spatial filling factor of the magnetic field.