Modeling YSO Jets in 3D II: Accretion-Fed, Star-Anchored Poynting Jets in the Low-Density Polar Cavity Powered by Disk-Magnetosphere Interaction

TL;DR

This work demonstrates that in 3D non-ideal MHD simulations of a YSO system, including a rotating stellar magnetosphere along with disk magnetic fields, a fast, lightly mass-loaded Poynting jet can be launched from the magnetosphere–disk interface. The jet originates from a rapid, load-fire-reload cycle on two-legged field lines anchored to the star and the elevated disk, generating toroidal magnetic pressure that accelerates gas within a low-density polar cavity, while reconnection reconfigures field lines to sustain the outflow. A slower, denser disk wind occupies the surrounding regions, and the stellar magnetic flux opening provides a polar backbone that enables persistent bipolar jets. The results highlight the robustness of magnetically driven, accretion-fed jets in magnetized YSOs and reveal how star–disk magnetic coupling shapes jet morphology, energetics, and angular-momentum transport, with implications for spin evolution and outflow observables. Higher-resolution studies are encouraged to further resolve the reconnection dynamics and mass loading in 3D without imposed diffusivity.</br>

Abstract

The origin of jets in young stellar objects (YSOs) remains a subject of active investigation. We present a 3D magnetohydrodynamic simulation of jet launching in YSOs, focusing on the interaction between the stellar magnetosphere and the accretion disk. In our model, a fast, low-density bipolar jet is powered by disk-magnetosphere interaction and launched through the polar cavity that is mass-loaded from the disk rather than the star. Specifically, outflows are driven by toroidal magnetic pressure generated along "two-legged" field lines, anchored at a magnetically dominated stellar footpoint and a mass-dominated point on the (magnetically elevated) disk surface via a cyclic "load-fire-reload" process: in the "load" stage, differential rotation between stellar and disk footpoints generates toroidal magnetic pressure; in the "fire" stage, vertical gradients in the toroidal field accelerate plasma and transport Poynting flux into the polar cavity; in the "reload" stage, magnetic reconnection allows the cycle to repeat, reforming "two-legged" field lines. These field lines are not required to be fully reset to a dipolar loop configuration; it is only required that the disk-end be shallowly embedded in the (elevated) disk surface. This rapid, asynchronous process produces a continuous, large-scale outflow. The resulting magnetically dominated (Poynting) jet, accelerated by magnetic pressure within the low-density polar cavity, is distinct from the denser, slower disk wind launched through the classic magnetic-tower mechanism. Comparison with a disk-only model shows that the rotating stellar magnetosphere promotes bipolar jet launching by shaping a magnetic geometry favorable to symmetric outflows.

Figures (14)

• Figure 1: 3D rendition of the bipolar jet launched in our model. The outer red and blue shells are isosurface contours at $100$ km s$^{-1}$, whereas the inner ones are at 200 km s$^{-1}$. The gray structure on the equatorial plane represents the disk. For reference, the green and yellow lines each have a length of 5 au, illustrating the physical scale of the system. An animated version of this figure can be found at: https://figshare.com/s/67335cb1846eca1edcb1. The movie is 27 seconds long, highlighting the strong bipolar outflow from the simulation's beginning (0 yr) to the end (5.5 yr).
• Figure 2: Left panel: Magnetic field geometry, represented by the time- and azimuthal-averaged magnetic field lines during the later stages of the simulation. The background color map shows the projected $\hat{z}$-direction velocity $v_z^p$ (equ. \ref{['equ:vzp']}). Right panel: a cartoon illustration of the magnetic field geometry, and key components in the simulation. Both panels: The magnetic field structure can be roughly divided into four distinct zones, delineated by corresponding magenta, cyan, and green lines in both the left and right panels. The magenta line encloses the permanently closed stellar magnetosphere; the cyan line marks the boundary of opened stellar field lines that still thread the star; the green line outlines the extent of the large-scale magnetic field lines threading the disk. The region between the cyan and green lines corresponds to a reconnecting, outflow-driving zone where the interaction between stellar and disk fields powers the outflow.
• Figure 3: Magnetic flux evolution in the simulation. The left panel shows the total magnetospheric flux (black line), defined as the maximum magnitude of the total downward $\hat{z}$-directed magnetic flux integrated azimuthally at each cylindrical radius. The red and blue lines indicate the amount of opened stellar magnetosphere flux, measured as the maximum magnitude of azimuthally-integrated downward $B_z$ at $z=\pm5$ au above and below the disk midplane, respectively. The right panel shows the time evolution of the enclosed magnetic flux at each cylindrical radius at the midplane. The stellar magnetosphere primarily contributes to the negative flux at small radii, while the disk field dominates the positive flux at larger radii.
• Figure 4: Overview of the outflow, particularly the disk wind at a large cylindrical radius surrounding the jet. Panel (a) and (b) show the density and projected $\hat{z}$-direction velocity ($v_z^p$, equ. \ref{['equ:vzp']}) respectively, overplotted by the azimuthal averaged fast-magnetosonic surface (the thick solid line); panel (c) shows the normalized MRI-turbulence wavelength (equ. \ref{['equ:mri_q']}; panel (d) and (e) show the plasma-$\beta$ and (equ. \ref{['equ:pbeta']}) kinetic-$\beta$ (equ. \ref{['equ:kbeta']}) respectively. The lower panels compare different force components in the disk wind. Panel (f) shows the ratio between the cylindrical-$\hat{R}$ direction magnetic force and centrifugal force. Panel (g), (h), (i), and (j) show the magnitude of the gas acceleration due to magnetic forces. The quantities are total gas acceleration in cylindrical-$\hat{R}$ direction, total magnetic acceleration in $\hat{z}$- direction, the effective magnetic pressure component (equ. \ref{['equ:az_mag_pres_eff']}), and the effective magnetic tension component (equ. \ref{['equ:az_mag_tens_eff']}) respectively. These disk wind properties and forces show that the disk wind in this model is very similar to the disk wind in Tu2025b.
• Figure 5: A zoom-in view focusing on the permanently closed stellar magnetosphere (around the midplane) and the opened stellar magnetic field threading the star (in the polar region above and below the star). Panel (a) shows the real density traced by the "passive scalars", highlighting the magnetospheric accretion streams. The sphere in the middle is the inner boundary $r_\mathrm{fix}$, where a star resides (see sec. \ref{['sec:method']} for a more detailed description of the setup). Panels (b) and (c) show the $B_z$ field strength and $v_z^p$ (equ. \ref{['equ:vzp']}) respectively, illustrating the magnetic and dynamical properties of the region close to the star. Panels (d) and (e) show the plasma-$\beta$ (equ. \ref{['equ:pbeta']}) and kinetic-$\beta$ (equ. \ref{['equ:kbeta']}) respectively, indicating the magnetosphere and the polar cavity are both magnetically dominated.