Observational Perspectives

Abstract

The slow rotation of T Tauri stars was an unexpected observational result not predicted by theory. The observations pose particular problems for models in which T Tauri stars are actively accreting substantial amounts of mass from circumstellar disks. Stellar winds have been invoked to carry off the required angular momentum, but the rapid rotation of a Persei and Pleiades stars implies that stellar wind angular momentum loss is relatively inefficient. The newest model suggests that interaction of a T Tauri magnetosphere with the disk limits the spinup of the star, in analogy with pulsars. Direct observational tests of this model are suggested. If magnetosphere-disk interactions last somewhat longer than a few million years, it may be possible to prevent some stars from spinning up during contraction to the main sequence, which would help explain the large dispersion in rotational velocities of low-mass stars seen in clusters like the Pleiades and a Per. It seems quite possible that low-mass stars may have differing rotational histories, and in particular that the Sun may never have been rapidly rotating.