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

Abstract

We perform three-dimensional (3D) 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 3D effects, we obtain a scaling relation of the magnetospheric radius very similar to the Ghosh & Lamb relation from the steady angular momentum transport equation.

Three-dimensional, Time-dependent MHD Simulation of Disk-Magnetosphere-Stellar Wind Interaction in a T Tauri, Protoplanetary System

“…Instead, it seems more likely that disk accretion happens in an episodic manner (see review by Audard et al 2014). This behavior has been demonstrated in a number of dynamic, multi-dimensional simulations (summarized in Romanova & Owocki 2015) and also (e.g., Orlando et al 2013;Colombo et al 2019;Vorobyov et al 2020;Zhu et al 2020;Martel & Lesur 2022;Takasao et al 2022). It is therefore desirable to relate the short-term disk-magnetosphere dynamics, which can be probed by observations, to the parameters that describe the long-term disk evolution, i.e., the average mass and angular momentum loss rates.…”

“…In order to estimate mass and angular momentum (AM) transfer in our simulation, we extract a number of spherical shells from the region between the disk and the stellar surface. Unlike some previous simulations (e.g., Takasao et al 2022), we avoid estimating these parameters exactly at the inner boundary itself, since their values may be affected by the rather complex boundary conditions the model uses. To get a good coverage of the different loss rates at different locations, we estimate these loss rate on sphere at r=2, 3, 5, and 7R .…”

“…To date, no numerical model for the diskmagnetosphere interaction has accounted for the effect of a self-consistent hot corona and accelerated stellar wind. The magnetospheric plasma is typically specified with some initial and boundary conditions, where a weak thermally driven wind (Parker 1958) may develop due to the buildup of a pressure gradient between the inner boundary and the simulation domain (e.g., Matt & Pudritz 2008;Takasao et al 2022). It is therefore important to account for the impact of the energized corona on the "disk-corona interaction", not "disk-magnetosphere interaction", where one should expect that the interaction will release some of the stored energy.…”

Three-dimensional, Time-dependent MHD Simulation of Disk–Magnetosphere–Stellar Wind Interaction in a T Tauri, Protoplanetary System

“…Instead, it seems more likely that disk accretion happens in an episodic manner (see review by Audard et al 2014). This behavior has been demonstrated in a number of dynamic, multidimensional simulations (summarized in Romanova & Owocki 2015and also, e.g., Orlando et al 2013;Colombo et al 2019;Vorobyov et al 2020;Zhu et al 2020;Martel & Lesur 2022;Takasao et al 2022). It is therefore desirable to relate the short-term disk-magnetosphere dynamics, which can be probed by observations, to the parameters that describe the long-term disk evolution, i.e., the average mass and angular momentum loss rates.…”

“…To date, no numerical model for the disk-magnetosphere interaction has accounted for the effect of a self-consistent hot corona and accelerated stellar wind. The magnetospheric plasma is typically specified with some initial and boundary conditions, where a weak thermally driven wind (Parker 1958) may develop due to the buildup of a pressure gradient between the inner boundary and the simulation domain (e.g., Matt & Pudritz 2008;Takasao et al 2022). It is therefore important to account for the impact of the energized corona on the "diskcorona interaction," not "disk-magnetosphere interaction," where one should expect that the interaction will release some of the stored energy.…”