Pranab Ghosh

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, g_cor, 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.