PREPRINT

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

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

Submitted on 2 November 2022

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.

Preprint

Comment: 38 pages, 21 figures. Accepted for publication in ApJ

Subjects: Astrophysics - Solar and Stellar Astrophysics; Astrophysics - Earth and Planetary Astrophysics; Astrophysics - High Energy Astrophysical Phenomena

URL: http://arxiv.org/abs/2211.01072

Accretion and ejection structures of Model A at $t=194.7$ day. Top row: azimuthally averaged data. Solid lines with arrows in the plasma $\beta$ map denote poloidal magnetic field lines, while those in the $v_r/v_{\rm esc}$ map show streamlines. Bottom row: data sliced at $\varphi=0$. From left to right, the density, the plasma $\beta$, the temperature, and the radial component of the velocity normalized by the local escape velocity, $v_r/v_{\rm esc}$.