Three-dimensional Simulations of Magnetospheric Accretion in a T Tauri Star: Accretion and Wind Structures Just Around Star

TL;DR

This study uses 3D magnetohydrodynamic simulations to explore accretion and wind structures near T Tauri stars, revealing complex multi-column accretion, angular momentum removal by winds, and weak dependence of magnetospheric radius on stellar spin.

Contribution

The paper presents novel 3D MHD simulation results showing the detailed structure of accretion and winds, and challenges classical theories on the influence of stellar spin on magnetospheric radius.

Findings

Multi-column accretion consistent with observations.

Significant angular momentum removal by winds reduces accretion torque.

Magnetospheric radius weakly depends on stellar spin, aligning with a modified Ghosh & Lamb relation.

Abstract

We perform three-dimensional magnetohydrodynamic simulations of magnetospheric accretion in a T Tauri star to study the accretion and wind structures in the close vicinity of the star. The gas accreting onto the star consists of the gas from the magnetospheric boundary and the failed disk winds. The accreting gas is commonly found as a multi-column accretion, which is consistent with observations. A significant fraction of the angular momentum of the accreting flows is removed by the magnetic fields of conical disk winds and turbulent failed winds inside and near the magnetosphere. As a result, the accretion torque is significantly reduced compared to the simple estimation based on the mass accretion rate. The stellar spin affects the time variability of the conical disk wind by changing the stability condition of the magnetospheric boundary. However, the time-averaged magnetospheric radius only weakly depends on the stellar spin, which is unlike the prediction of classical theories that the stellar spin controls the magnetospheric radius through the magnetic torque. The ratio of the toroidal to the poloidal field strengths at the magnetospheric boundary, which is a key parameter for the magnetic torque, is also insensitive to the spin; it is rather determined by the disk dynamics. Considering newly found three-dimensional effects, we obtain a scaling relation of the magnetospheric radius very similar to the Ghosh & Lamb relation from the steady angular momentum transport equation.