Gas dynamics around dust asymmetries in turbulent disks

Abstract

High-resolution ALMA observations have revealed asymmetric dust crescents in several protoplanetary disks, suggesting efficient dust trapping mechanisms potentially linked to gas vortices. While such features have been associated with vortices--whether induced by massive planets, turbulence , or other disk processes--their origin remains unclear. In this study, we investigate the viability of dust trapping by vortices that are self-sustained in disks dominated by Vertical Shear Instability (VSI) turbulence. We perform 3D hydrodynamic simulations using the PLUTO code with Lagrangian particles of three sizes (1 mm, 500~$μ$m, 100~$μ$m) to analyze the gas-dust dynamics around vortices. Our simulations reveal the formation of multiple vortices, including two characteristic large-scale, long-lived vortices that are able to capture the dust particles. We also find that dust vertical diffusion is reduced within vortices, suggesting that these structures preferentially enhance radial and azimuthal motions. Finally we generate synthetic dust continuum images at different wavelength bands and velocity residuals to compare the observable properties with ALMA observations. No clear spiral features are observed in either the synthetic dust images or the velocity residuals, unlike in vortices triggered by planets. Projection effects at high disk inclinations can obscure dust asymmetries, implying that more disks may host dust crescents than currently reported.

Figures (18)

• Figure 1: Reynolds stress-to-pressure ratio, $\alpha_{r,\phi}$. The top panel shows the time-averaged $\alpha_{r,\phi}$, where the yellow line represents the convergence value, $\langle \alpha_{r,\phi} \rangle = 3\times10^{-4}$. For reference, the vertical dotted line corresponds to the outer local orbit at 0.25$R_{0}$ used by Lesur_2025 in their analysis, as shown in their Figure 1. The bottom-left panel displays the time series of $\alpha_{r,\phi}$ in r-direction, averaged over azimuth in the 3D VSI-active disk. The bottom-right panel presents the radial Reynolds stress-to-pressure ratio over the time series and averaged in the azimuthal directions.
• Figure 2: Vorticity of the gas in the midplane in units of the keplerian frequency at 350 orbits. Overplotted are two dust-to-gas mass ratio values of 1 mm particles in the midplane.
• Figure 3: Time evolution of the minimum vorticity residual as a function of radius, illustrating the radial position and migration of vortices in the VSI-unstable disk. The horizontal dashed lines outline the final locations of the two large scale vortices.
• Figure 4: Pertubational azimuthal velocity (top and bottom left panels), vertical velocity (top and bottom middle panels) of the disk at the midplane ($z=0$, top panels), and close to the disk surface ($z>2H$, bottom panels). To make it easier to identify the two large-scale vortices in the disk, we show the dust distribution vertically averaged in the top and bottom right panels. The black arrows point at the location of the inner and outer vortices.
• Figure 5: Velocity profile of a single 100 $\mu$m-sized particle (left) and a 1 mm-sized particle (right) within the vortex.