Rotation of T Tauri stars: accretion discs and stellar dynamos

Abstract

Observed rotation periods of T Tauri stars (TTS) lie in the range |$1\lesssim P$| (days) |$\lesssim 10$|⁠, with a ‘fast rotator’ group centred at |$P\sim 3$| d, and a ‘slow rotator’ group centred at |$P\sim 8$| d. I show here that such periods are a natural consequence of the tendency of spin-down accretion torques to regulate the stellar rotation rates to values near the equilibrium rates determined by the interaction between circumstellar accretion discs and dynamo-generated magnetic fields of these stars. According to the theory sketched here, in a plot of stellar rotation periods against a parameter g, which depends on stellar properties and the accretion rate, TTS are expected to lie at or below a ‘spin-down line’. In a more approximate version of the theory, which utilizes an empirical correlation between the masses and accretion rates of TTS, the controlling parameter, gcor, depends on stellar properties alone. Predictions from both theories are in agreement with observations. I discuss the role of both classical and weak-lined TTS in this view of pre-main-sequence angular momentum regulation.