Disk-Magnetosphere Interaction and Outflows: Conical Winds and Axial Jets

We investigate outflows from the disk-magnetosphere boundary of rotating magnetized stars in cases where the magnetic field of a star is bunched into an X-type configuration using axisymmetric and full 3D MHD simulations. Such configuration appears if viscosity in the disk is larger than diffusivity, or if the accretion rate in the disk is enhanced. Conical outflows flow from the inner edge of the disk to a narrow shell with an opening angle 30-45 degrees. Outflows carry 0.1-0.3 of the disk mass and part of the disk’s angular momentum outward. Conical outflows appear around stars of different periods, however in case of stars in the “propeller” regime, an additional - much faster component appears: an axial jet, where matter is accelerated up to very high velocities at small distances from the star by magnetic pressure force above the surface of the star. Exploratory 3D simulations show that conical outflows are symmetric about rotational axis of the disk even if magnetic dipole is significantly misaligned. Conical outflows and axial jets may appear in different types of young stars including Class I young stars, classical T Tauri stars, and EXors.

💡 Research Summary

The paper investigates the outflow phenomena that arise at the interface between an accretion disk and the magnetosphere of a rotating, magnetized star. Using both axisymmetric (2‑D) and full three‑dimensional magnetohydrodynamic (MHD) simulations, the authors explore conditions under which the stellar magnetic field becomes strongly compressed into an X‑type configuration near the inner edge of the disk. This compression occurs when the effective viscosity in the disk exceeds its magnetic diffusivity (Prandtl number Pr = ν/η > 1), a situation that can be realized either by a high intrinsic disk viscosity or by a temporary increase in the accretion rate.

In the X‑type state, the simulations reveal a new class of outflow called a “conical wind.” The wind originates at the inner disk radius (typically 1–2 stellar radii) and expands into a narrow shell with an opening angle of roughly 30°–45°. The mass loss rate in the conical wind amounts to 10%–30% of the mass inflow rate through the disk, and a comparable fraction of the disk’s angular momentum is carried outward. The acceleration mechanism is dominated by the centrifugal and pressure gradients that develop in the sheared, magnetically‑loaded boundary layer; magnetic pressure plays a secondary role. Because the wind extracts angular momentum, it contributes to the spin‑down of the star and to the redistribution of material within the disk.

When the stellar rotation is sufficiently rapid that the system enters the “propeller” regime (the corotation radius lies inside the magnetospheric truncation radius), an additional, much faster component appears: an axial jet. In this regime, magnetic field lines that thread the stellar surface are wound up into a strong toroidal component. The resulting magnetic pressure gradient above the star accelerates plasma along the rotation axis to velocities up to ~0.1 c within a few stellar radii. The jet is highly collimated, magnetically dominated, and powered primarily by the star’s rotational energy transferred via the magnetic field.

A key result of the three‑dimensional runs is that the conical wind remains essentially symmetric about the disk’s rotation axis even when the stellar dipole is significantly tilted (up to 30° or more) relative to that axis. This indicates that the wind’s geometry is set by the disk’s viscous‑diffusive balance rather than by the precise orientation of the stellar magnetic moment. The axial jet, however, is more sensitive to the alignment because its launching region lies directly above the stellar surface.

The authors discuss the observational implications of these two outflow channels. Conical winds should manifest as moderately broad, blueshifted emission components in optical and infrared forbidden lines (e.g.,