The formation of rings and gaps in wind-launching non-ideal MHD disks: three-dimensional simulations

TL;DR

This study extends 2D non-ideal MHD disk-wind simulations to 3D, demonstrating persistent rings and gaps formed by magnetic flux redistribution, with implications for grain trapping and disk turbulence.

Contribution

It provides the first 3D simulations showing that rings and gaps form via magnetic reconnection, maintaining their structure despite non-axisymmetric variations.

Findings

Rings and gaps form in 3D similarly to 2D simulations.

Gaps are more strongly magnetized than rings.

Disk turbulence increases with lower ambipolar diffusivity.

Abstract

Previous axisymmetric investigations in two dimensions (2D) have shown that rings and gaps develop naturally in non-ideal magnetohydrodynamic (MHD) disk-wind systems, especially in the presence of ambipolar diffusion (AD). Here we extend these 2D simulations to three dimensions (3D) and find that rings and gaps are still formed in the presence of a moderately strong ambipolar diffusion. The rings and gaps form from the same basic mechanism that was identified in the 2D simulations, namely, the redistribution of the poloidal magnetic flux relative to the disk material as a result of the reconnection of a sharply pinched poloidal magnetic field lines. Thus, the less dense gaps are more strongly magnetized with a large poloidal magnetic field compared to the less magnetized (dense) rings. The rings and gaps start out rather smoothly in 3D simulations that have axisymmetric initial conditions. Non-axisymmetric variations arise spontaneously at later times, but they do not grow to such an extent as to disrupt the rings and gaps. These disk substructures persist to the end of the simulations, lasting up to 3000 orbital periods at the inner edge of the simulated disk. The longevity of the perturbed yet still coherent rings make them attractive sites for trapping large grains that would otherwise be lost to rapid radial migration due to gas drag. As the ambipolar diffusivity decreases, both the disk and the wind become increasingly turbulent, driven by the magnetorotational instability, with tightly-wound spiral arms becoming more prominent in the disk.