  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. Sunspots
are believed to result from the eruption of magnetic flux-tubes rising from the
deep interior of the Sun. The strong magnetic field locally reduces convective
heat transport to the solar surface. Such flux-tube models have also 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. Here we show that direct numerical simulations of a distributed
dynamo operating in a density-stratified rotating spherical shell can
spontaneously generate cool spots. Convection in the interior of the model
produces a large scale magnetic field which interacts with near surface
granular convection leading to strong concentrations of magnetic flux and
formation of starspots. Prerequisites for the formation of sizeable
high-latitude spots in the model are sufficiently strong density stratification
and rapid rotation. Our model presents an alternate mechanism for starspot
formation by distributed dynamo action.
