SOLAR SPIN-DOWN WITH INTERNAL MAGNETIC-FIELDS

We investigate the rotational evolution of a solar-type star containing a large-scale poloidal magnetic field in its radiative core, in response to the torque applied to it by magnetically coupled wind. Our model takes into account both the generation of a toroidal magnetic component via shearing of the existing poloidal component by differential rotation, as well as the back-reaction on the differential rotation due to Lorentz forces associated with the toroidal field. Our computations demonstrate the existence of classes of large-scale poloidal magnetic fields allowing rapid spin-down of the surface layers shortly after the arrival on the zero-age main sequence, while producing weak internal differential rotation in the radiative core by the solar age. This indicates that the constraints brought about by rotational evolution of solar analogs in young clusters and by helioseismology are not incompatible with the existence of large-scale magnetic fields in stellar radiative interiors. The present surface solar rotation rate is also shown to be a poor indicator of the strength and geometry of hypothetical poloidal magnetic fields pervading the solar radiative interior.