Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars

Context. Observations of cool stars reveal dark spot-like features on their surfaces. Compared to sunspots, starspots can be bigger or cover a larger fraction of the stellar surface. While sunspots appear only at low latitudes, starspots are also found in polar regions, in particular on rapidly rotating stars. Conventional flux-tube models have been invoked to explain starspot properties. However, these models use several simplifications and so far the generation of either sunspots or starspots has not been demonstrated in a self-consistent simulation of stellar magnetic convection. Aims. To clarify conditions for the spontaneous formation of dark spots in numerical models of convection-driven stellar dynamos. Methods. We simulate convection and magnetic field generation in rapidly rotating spherical shells under the anelastic approximation. The high-resolution simulations were performed using a fully-spectral magnetohydrodynamic code. Results. We demonstrate for the first time that a self-consistent distributed dynamo can spontaneously generate high latitude dark spots. Dark spots are generated when large-scale magnetic field, generated in the bulk of the convection zone, interacts with and locally quenches flow near the surface. Prerequisites for the formation of sizeable dark spots in the model are sufficiently strong density stratification and rapid rotation. Conclusions. Our models present an alternative scenario for starspot formation by distributed dynamo action. Our results also lend strong support to the idea that dynamos in the interiors of rapidly rotating stars might be fundamentally different from the solar one.