Magnetic flux emergence in fast rotating stars

Abstract

Fast rotating cool stars are characterised by high magnetic activity levels and frequently show dark spots up to polar latitudes. Their distinctive surface distributions of magnetic flux are investigated in the context of the solar-stellar connection by applying the solar flux eruption and surface flux transport models to stars with different rotation rates, mass, and evolutionary stage. The rise of magnetic flux tubes through the convection zone is primarily buoyancy-driven, though their evolution can be strongly affected by the Coriolis force. The poleward deflection of the tube's trajectory increases with the stellar rotation rate, which provides an explanation for magnetic flux eruption at high latitudes. The formation of proper polar spots likely requires the assistance of meridional flows both before and after the eruption of magnetic flux on the stellar surface. Since small radiative cores support the eruption of flux tubes at high latitudes, low-mass pre-main sequence stars are predicted to show high mean latitudes of flux emergence. In addition to flux eruption at high latitudes, main sequence components of close binary systems show spot distributions which are non-uniform in longitude. Yet these `preferred longitudes' of flux eruption are expected to vanish beyond a certain post-main sequence evolutionary stage.