Magnetic braking of T Tauri stars

We construct models for the rotation rates of T Tauri stars whose spin isregulated by magnetic linkage between the star and a surrounding accretiondisc. Our models utilise a time-dependent disc code to follow the accretionprocess and include the effects of pre-main-sequence stellar evolution. We findthat the initial disc mass controls the evolution of the star-disc system. Forsufficiently massive discs, a stellar field of $\sim$ 1 kG is able to regulatethe spin rate to the observed values during the classical T Tauri phase. Thefield then acts to expel the disc and the star spins up at constant angularmomentum as a weak-line system. Lower mass discs are ejected at an early epochand fail to brake the star significantly. We extend the model to close binarysystems, and find that the removal of angular momentum from the disc by thesecondary significantly prolongs the inner disc lifetime. Such systems shouldtherefore be relatively slow rotators. We also discuss the implications of ourmodel for the spectral energy distributions and variability of T Tauri stars.Comment: 10 pages of uu-encoded gz-compressed Postscript, including 9 figures. MNRAS in pres