Magnetic Braking of T Tauri Stars

We construct models for the rotation rates of T Tauri stars whose spin is regulated by magnetic linkage between the star and a surrounding accretion disc. Our models utilise a time-dependent disc code to follow the accretion process and include the eeects of pre-main-sequence stellar evolution. We nd that the initial disc mass controls the evolution of the star-disc system. For suuciently massive discs, a stellar eld of 1 kG is able to regulate the spin rate to the observed values during the classical T Tauri phase. The eld then acts to expel the disc and the star spins up at constant angular momentum as a weak-line system. Lower mass discs are ejected at an early epoch and fail to brake the star signiicantly. We extend the model to close binary systems, and nd that the removal of angular momentum from the disc by the secondary signiicantly prolongs the inner disc lifetime. Such systems should therefore be relatively slow rotators. We also discuss the implications of our model for the spectral energy distributions and variability of T Tauri stars.