Magnetic torques on T Tauri stars: accreting vs. non-accreting systems

TL;DR

This study uses magnetohydrodynamic simulations to analyze how magnetic torques from accreting and ejected flows influence the rotational evolution of T Tauri stars, highlighting the role of magnetospheric ejections and stellar winds.

Contribution

It provides detailed torque scalings for different flow components in star-disk interactions, revealing the impact of magnetospheric ejections on stellar wind confinement and braking efficiency.

Findings

Magnetospheric ejections confine stellar wind expansion, shaping outflow geometry.

Stellar wind braking is more effective in star-disk-interacting systems than in isolated stars.

Stellar winds extract less than 2% of the mass accretion rate in simulations.

Abstract

Classical T Tauri stars (CTTs) magnetically interact with their surrounding disks, a process that is thought to regulate their rotational evolution. In this work, we compute torques acting onto the stellar surface of CTTs arising from different accreting (accretion funnels) and ejecting (stellar winds and magnetospheric ejections) flow components. Furthermore, we compare the magnetic braking due to stellar winds in two different systems: isolated and accreting stars. 2.5D magnetohydrodynamic, time-dependent, axisymmetric simulations are employed. For both systems the stellar wind is thermally driven. In the star-disk-interaction (SDI) simulations the accretion disk is Keplerian, viscous, and resistive. Two series of simulations are presented, one for each system. We find that in classical T Tauri systems the presence of magnetospheric ejections confines the stellar-wind expansion, resulting in an hourglass-shaped geometry of the outflow. In addition, the formation of the accretion columns modifies the amount of open magnetic flux exploited by the stellar wind. These effects have a strong impact on the stellar wind properties and we show that the stellar-wind braking is more efficient in the star-disk-interacting systems than in the isolated ones. We also derive torque scalings, over a wide range of magnetic field strengths, for each flow component in a SDI system that directly applies a torque on the stellar surface. In all the performed SDI simulations the stellar wind extracts less than 2% of the mass accretion rate and the disk is truncated up to 66% of the corotation radius. All the simulations show a net spin-up torque. In order to achieve a stellar-spin equilibrium we need either more massive stellar winds or disks being truncated closer to the corotation radius, which increases the torque efficiency by the magnetospheric ejections.