Why rapid rotators have polar spots.

Starspots on magnetically active, cool stars preferentially appear near the poles. We suggest that this preference of high latitudes is due to the rapid rotation to these stars which leads to a dominance of the Coriolis force over the buoyancy force in the dynamics of magnetic flux tubes. As a consequence, flux tubes erupting from the deep parts of the stellar convection zone follow a path nearly parallel to the axis of rotation and thus necessarily surface at high latitudes, unless their initial field strength exceeds a critical value for which buoyancy becomes dominant again. It is shown that for stars with rotation periods below about 10 days flux tubes with such large field strength (of the order of 10 exp 6 G) cannot be formed and stored since they are unstable with respect to non-axisymmetric disturbances. Consequently, magnetically active stars with rapid rotation exhibit magnetic flux eruption at high latitudes and polar starspots.