On disk braking of T Tauri rotation

The rotation of T Tauri stars is thought to be slowed by coupling between the stellar magnetosphere and the circumstellar disk. Stassun and collaborators have recently questioned this picture, based on their observations of young stars in and near the Orion Nebula Cluster (ONC) that indicate little evidence for disk-mediated spin-down. However, stellar spin-down can occur no faster than the rate at which angular momentum is removed from the inner disk, either through viscous processes associated with accretion or by a wind. Observational estimates of these processes suggest that the timescales for angular momentum loss may be comparable to the ages of the ONC stars, so that disk braking may not be completely effective, at least among the lowest mass stars. Testing this hypothesis will require further measurements of mass accretion and mass-loss rates as well as determinations of rotational periods as a function of age.