SF-06-0011

3D MHD simulations of magnetospheric accretion in protostars and T Tauri stars

Shinsuke Takasao, Kengo Tomida, Kazunari Iwasaki, Takeru K. Suzuki, Masanobu Kunitomo

We perform three-dimensional magnetohydrodynamic simulations of magnetospheric accretion in a protostar and a T Tauri star and study the angular momentum transfer around the star. The simulations show that the accretion torque is significantly smaller than expected in previous theories. We find that the angular momentum of the accreting gas is efficiently extracted by different disk winds. In addition to the conical disk wind, the turbulent magnetospheric wind, which can appear only in three-dimension, plays an important role even when the conical disk wind is absent. The magnetosphere has been assumed to rotate rigidly with the star in previous theories, but our simulations find that a large portion of the magnetosphere rotates nearly at the local Keplerian velocity. The accreting gas penetrating the magnetosphere forces it to rotate at such a speed. Considering newly found three-dimensional effects, we derive an equation of the magnetospheric radius very similar to the Ghosh and Lamb relation from the steady angular momentum transport equation. We also discuss the spin-down of a protostar in the magnetospheric accretion paradigm. We argue that a significant reduction in the spin-up torque by different disk winds is critical for an efficient stellar spin-down.