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

Abstract

Context. Classical T Tauri stars (CTTs) magnetically interact with their surrounding disks, a process that is thought to regulate their rotational evolution. Aims. 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. Besides, we compare the magnetic braking due to stellar winds in two different systems: isolated (i.e., weak-line T Tauri and main-sequence) and accreting (i.e., classical T Tauri) stars. Methods. We use 2.5D magnetohydrodynamic, time-dependent, axisymmetric simulations, computed with the PLUTO code. For both systems the stellar wind is thermally driven. In the star-disk-interaction (SDI) simulations the accretion disk is Keplerian, viscous, and resistive, modeled with an alpha prescription. Two series of simulations are presented, one for each system (i.e., isolated and accreting stars). Results. In classical T Tauri systems the presence of magnetospheric ejections confines the stellar-wind expansion, resulting in a hourglass-shaped geometry of the outflow and 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 further derive torque scalings, over a wide range of magnetic field strengths, for each flow component in a star-disk-interacting system (i.e., magnetospheric accretion/ejections, stellar winds), which directly applies a torque on the stellar surface. Conclusions. 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 simulations show a net spin-up torque. We conclude that in order to achieve a stellarspin equilibrium we need either more massive stellar winds or disks being truncated closer to the corotation radius, which increases the torque efficiency of the magnetospheric ejections.

Magnetic braking of accreting T Tauri stars: Effects of mass accretion rate, rotation, and dipolar field strength

“…This is a steeper scaling compared to the analytical power-law index (𝑚 t = 2/7), due to the fact that the accretion disk perturbs the magnetosphere. Using an identical set up, Pantolmos et al (2020) find 𝐾 t = 0.704 (cor-rected for polar magnetic field strength) and 𝑚 t = 0.350 for their 𝐵 ★ parameter study, showing close agreement with our determined best-fit dimensionless parameters. Gallet et al (2019) assume a dipolar magnetosphere and use 𝐾 t = 0.766 and 𝑚 t = 2/7 for their 𝑅 t parameterization (corrected for polar magnetic field strength), thus, we predict higher truncation radii than those predicted by Gallet et al (2019) for our parameter range.…”

“…where 𝑢(𝑇) is specific internal energy as a function of temperature 𝑇, determined via our new adopted caloric equation of state for a calorically imperfect gas, where the specific heats and adiabatic index 𝛾 are temperature-dependent; this is described in detail in the appendix of Pantolmos et al (2020). The hot stellar wind plasma will behave nearisothermally (𝛾 = 1.05), and the cold accretion disk will behave adiabatically (𝛾 = 5/3).…”

“…This avoids misclassifying field lines that are still closed but are strongly deformed due to MEs (and in practice, these strongly deformed field lines open and close intermittently, while the lines we select typically remain closed for the duration). The truncation radius is then taken to be the radial coordinate corresponding to the minimum specific entropy (which locates the densest region of the disk material) along this last closed field line (Pantolmos et al 2020).…”

“…Therefore, we predict a higher spin-up accretion torque for a given SDI case in our parameter regime. Pantolmos et al (2020) adopted a fixed 𝐾 acc = 0.790 with no additional dependence on 𝑅 t , predicting an even higher accretion torque than our formulation; however, this value was determined using a smaller set of simulations, solely changing 𝐵 ★ .…”

“…Our stellar ME torque formulation offers additional selfconsistency to the prescription introduced in Gallet et al (2019), and adopted by Pantolmos et al (2020). Firstly, Gallet et al (2019) use the term 𝐾 rot to account for the sub-Keplerian correction to the magnetic field twist (or the stardisk differential rotation).…”