MAGNETIC BRAKING IN SPIN EVOLUTION OF MAGNETIZED T-TAURI STARS

The spin rates of classical T Tauri stars (CTTS) are an order of magni tude smaller than the break-up rates despite their relatively long per iods of mass accretion. We examine the idea of magnetic braking as a p ossible explanation for slow spin rates. During the pre-main-sequence Hayashi phase, a large number of combinations of plausible initial con ditions lead to sufficiently small spin rates accounting for the obser ved slow spin rates. We show that this conclusion depends only weakly on type of stellar magnetic fields, time evolution of mass accretion r ates, and initial conditions. The results suggest that the observed sl ow spin rates of CTTS may indeed be due to disk-star magnetic coupling and braking. We derive a simple constraint on strengths of initial (o r constant) magnetic fields required for slow final spin rates. In the constant field case or in the evolving dynamo field case, an initial field strength B > a few hundred gauss is sufficient to achieve the fi nal slow quasi-equilibrium spin rates within approximately 10(6) yr. T his conclusion is valid unless the initial spin rates are very close t o the break-up rates. The derived lower limits on B are consistent wit h those invoked for the Alfven winds.