Exploring the Role of a Tachocline in M-Dwarf Magnetism

Abstract

M-type stars are quickly stepping into the forefront as some of the best candidates in searches for habitable Earth-like exoplanets, and yet many M-dwarfs exhibit extraordinary flaring events which would bombard otherwise habitable planets with ionizing radiation. In recent years, observers have found that the fraction of M-stars demonstrating significant magnetic activity transitions sharply from roughly $10\%$ for main-sequence stars earlier (more massive) than spectral type M3.5 (0.35 M$_\odot$) to nearly $90\%$ for stars later than M3.5. Suggestively, it is also later than M3.5 at which main-sequence stars become fully convective, and may no longer contain a tachocline. Using the spherical 3D MHD simulation code Rayleigh, we compare the peak field strengths, topologies, and time dependencies of convective dynamos generated within a quickly rotating (2 $\Omega_\odot$) M2 (0.4 M$_\odot$) star, with the computational domain either terminating at the base of the convection zone or including the tachocline. We find that while both models generate strong ($\sim$10kG), wreathlike toroidal fields exhibiting polarity reversals, the tachocline model provided a further reservoir for the toroidal field, which slowed the average reversal period from 100 rotations to more than 220 rotations and increased the spectral power of the low-order modes of the near-surface radial field by a factor of 4.