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

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: yadav@mps.mpg.de
² Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany

Received: 11 July 2014
Accepted: 23 November 2014

Abstract

Context. Observations of cool stars reveal dark spot-like features on their surfaces. These starspots can be more extended than sunspots and cover a large area 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, neither sunspots nor starspots have been generated in a self-consistent simulation of stellar magnetic convection.

Aims. We aim to clarify the conditions necessary for the spontaneous formation of dark spots in numerical models of convection-driven stellar dynamos.

Methods. We simulated convection and magnetic field generation in rapidly rotating spherical shells assuming 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 a large-scale magnetic field, generated in the bulk of the convection zone, interacts with and locally quenches flow near the surface. Sufficiently strong density stratification and rapid rotation are prerequisites for the formation of sizeable dark spots in the model.

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.