Probing the Influence of a Tachocline in Simulated M-dwarf Dynamos

Abstract

Abstract M-type stars are among the best candidates in searches for habitable Earth-like exoplanets, and yet many M-dwarfs exhibit extraordinary flaring that would bombard otherwise habitable planets with ionizing radiation. Observers have found that the fraction of M-stars demonstrating significant activity transitions from roughly 10% for main-sequence stars more massive than 0.35 M ⊙ to nearly 90% for less massive stars. The latter are typically rotating quite rapidly, suggesting differing spin-down histories. It is also below 0.35 M ⊙ when main-sequence stars become fully convective and may no longer contain a tachocline. We turn here to the more massive M-stars to study the impact such a layer may have on their internal dynamics. Using the global MHD code Rayleigh, we compare the properties of convective dynamos generated within rapidly rotating 0.4 M ⊙ stars, with the computational domain either terminating at the base of the convection zone or permitting overshoot into the underlying stable region. We find that a tachocline is not necessary for the organization of strong toroidal wreaths of magnetism in these stars, though it can increase the coupling of mean field amplitudes to the stellar rotation rate. Additionally, we note that the presence of a tachocline tends to make magnetic cycles more regular than they would otherwise have been, and can permit alternative field configurations with much longer cycles. Finally, we find that the tachocline helps enhance the emergent fields and organize them into larger spatial scales, providing favorable conditions for more rapid spin-down via the stellar wind.