Magnetic Braking of Accreting T Tauri Stars II: Torque Formulation Spanning Spin-up and Spin-down Regimes

Abstract

The magnetic interaction between a classical T Tauri star and its surrounding accretion disk is thought to influence its rotational evolution. We use 2.5D magnetohydrodynamic, axisymmetric simulations of star-disk interaction, computed via the PLUTO code, to calculate the net torque acting on these stars. We divide the net torque into three contributions: accretion (spin-up), stellar winds (spin-down), and magnetospheric ejections (MEs) (spin-up or down). In Paper I, we explored interaction regimes in which the stellar magnetosphere truncates the inner disk at a location spinning faster than the star, resulting in a strong net spin-up contribution from accretion and MEs (“steady accretion” regime). In this paper, we investigate interaction regimes in which the truncation radius gets closer to and even exceeds corotation, where it is possible for the disk material to gain angular momentum and be periodically ejected by the centrifugal barrier (“propeller” regime). This reduces the accretion torque, can change the sign of the ME torque, and can result in a net stellar spin-down configuration. These results suggest it is possible to have a net spin-down stellar torque even for truncation radii within the corotation radius (R t ≳ 0.7R co). We fit semi-analytic functions for the truncation radius, and the torque associated with star-disk interaction (i.e., the sum of accretion and ME torques) and stellar wind, allowing for the prediction of the net stellar torque for a parameter regime covering both net spin-up and spin-down configurations, as well as the possibility of investigating rotational evolution via 1D stellar evolution codes.

GRMHD simulations of accreting neutron stars with non-dipole fields

“…The theoretical r m v alues ho we ver use the lo wer Ṁ extracted at the stellar surface which results in an o v erestimation of the true r m . A similar effect was also observed by Ireland, Matt & Zanni ( 2022 ) in case of YSOs.…”

Tracking Hidden Magnetospheric Fluctuations in Accretion-powered Pulsars With a Kalman Filter

“…above scalings raise number of parameters from seven to 10. Equations (C15) and (C16) have been generalized semianalytically to interpolate between accretor (R m < R c ) and propeller (R m > R c ) regimes(Ireland et al 2022) and to model advection-dominated super-Eddington systems(Chashkina et al 2019) and plasmoid ejection(Zanni & Ferreira 2013).…”

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

“…A wealth of 2D simulations (e.g., Matt & Balick 2004;Matt & Pudritz 2008), and 2.5D simulations (e.g., Goodson et al 1997;Zanni & Ferreira 2009;Bai 2017;Béthune et al 2017;Pantolmos et al 2020;Ireland et al 2022) (Colombo et al 2019) investigated the transfer of mass and angular momentum from the disk to the star, and the different accretion patterns. A significant fraction of the three-dimensional MHD studies of diskmagnetosphere interaction in T Tauri systems was done using a model that was initially presented in Koldoba et al (2002) and Romanova et al (2002).…”

“…The ideal picture of a steady accretion through the large magnetospheric loops has been revised in more recent studies, which have looked into the opening of the stellar magnetic field lines by the rotating disk. These studies show a significant modification of the angular momentum transfer between the disk and the stellar corona compared to the picture of steady accretion along the assumed dipolar stellar field lines (Agapitou & Papaloizou 2000;Matt & Pudritz 2008;Zanni & Ferreira 2009;Bai 2017;Béthune et al 2017;Pantolmos et al 2020;Ireland et al 2022).…”