Formation of Dust Rings and Gaps in Non-ideal MHD Disks Through Meridional Gas Flows

TL;DR

This study uses 2D non-ideal MHD simulations to show how meridional gas flows in young stellar disks influence dust ring and gap formation, explaining observed substructures and dust distribution dynamics.

Contribution

It reveals the dominant role of meridional gas motions over dust-gas drift in shaping dust substructures in non-ideal MHD disks, supported by detailed simulation results.

Findings

Dust grains drift faster towards gas rings due to pressure gradients.

Meridional flows carry settled dust rapidly inward and upward.

Flow patterns explain observed gas motions in disks like HD 163296.

Abstract

Rings and gaps are commonly observed in the dust continuum emission of young stellar disks. Previous studies have shown that substructures naturally develop in the weakly ionized gas of magnetized, non-ideal MHD disks. The gas rings are expected to trap large mm/cm-sized grains through pressure gradient-induced radial dust-gas drift. Using 2D (axisymmetric) MHD simulations that include ambipolar diffusion and dust grains of three representative sizes (1~mm, 3.3~mm, and 1~cm), we show that the grains indeed tend to drift radially relative to the gas towards the centers of the gas rings, at speeds much higher than in a smooth disk because of steeper pressure gradients. However, their spatial distribution is primarily controlled by meridional gas motions, which are typically much faster than the dust-gas drift. In particular, the grains that have settled near the midplane are carried rapidly inwards by a fast accretion stream to the inner edges of the gas rings, where they are lifted up by the gas flows diverted away from the midplane by a strong poloidal magnetic field. The flow pattern in our simulation provides an attractive explanation for the meridional flows recently inferred in HD 163296 and other disks, including both "collapsing" regions where the gas near the disk surface converges towards the midplane and a disk wind. Our study highlights the prevalence of the potentially observable meridional flows associated with the gas substructure formation in non-ideal MHD disks and their crucial role in generating rings and gaps in dust.