Do Accretion Discs Regulate the Rotation Rates of Young Stars?

The question of the early evolution of stellar angular momentum is an interesting and essentially unsolved problem. It is well known that a substantial number of young stars show spin rates well below their break-up speed [4], despite the fact that these stars should spin up as they contract towards the main sequence. Furthermore, the angular momentum distributions of older stellar clusters such as the Pleiades, and the simultaneous presence of ultrafast rotators and slowly rotating stars amongst solar-type stars, argue for a phase of angular momentum regulation amongst pre-main-sequence stars [2, 17]. The most promising mechanism to regulate the angular momentum of young stars is disc-locking [9]. In the disc locking theory, magnetic field lines connect the star to the disc, enforcing synchronous rotation between the star and the material in the disc at some radius, near where the magnetic field disrupts the disc.