MAGNETIC BRAKING IN SPIN EVOLUTION OF MAGNETIZED T-TAURI STARS

The spin rates of classical T Tauri stars (CTTS) are an order of magnitude smaller than the break-up rates despite their relatively long periods of mass accretion. We examine the idea of magnetic braking as a possible explanation for slow spin rates. During the pre-main-sequence Hayashi phase, a large number of combinations of plausible initial conditions lead to sufficiently small spin rates accounting for the observed slow spin rates. We show that this conclusion depends only weakly on type of stellar magnetic fields, time evolution of mass accretion rates, and initial conditions. The results suggest that the observed slow spin rates of CTTS may indeed be due to disk-star magnetic coupling and braking. We derive a simple constraint on strengths of initial (or 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 final slow quasi-equilibrium spin rates within approximately 10(6) yr. This conclusion is valid unless the initial spin rates are very close to the break-up rates. The derived lower limits on B are consistent with those invoked for the Alfven winds.