Lopsided dust rings in transition disks

TL;DR

This paper investigates how dust accumulates in lopsided rings within transition disks, demonstrating that azimuthal density gradients can lead to observable dust concentrations, providing insights into early planet formation processes.

Contribution

The study introduces a 1+1D analytical method to model azimuthal dust accumulation, linking it to observable signatures and turbulence levels in transition disks.

Findings

Azimuthal dust gradients can cause strong, long-lived dust concentrations.

The effect is dependent on the Péclet number, balancing advection and diffusion.

Simulated ALMA observations can detect these dust accumulations.

Abstract

Context. Particle trapping in local or global pressure maxima in protoplanetary disks is one of the new paradigms in the theory of the first stages of planet formation. However, finding observational evidence for this effect is not easy. Recent work suggests that the large ring-shaped outer disks observed in transition disk sources may in fact be lopsided and constitute large banana-shaped vortices. Aims. We wish to investigate how effective dust can accumulate along the azimuthal direction. We also want to find out if the size- sorting resulting from this can produce a detectable signatures at millimeter wavelengths. Methods. To keep the numerical cost under control we develop a 1+1D method in which the azimuthal variations are treated sepa- rately from the radial ones. The azimuthal structure is calculated analytically for a steady-state between mixing and azimuthal drift. We derive equilibration time scales and compare the analytical solutions to time-dependent numerical simulations. Results. We find that weak, but long-lived azimuthal density gradients in the gas can induce very strong azimuthal accumulations of dust. The strength of the accumulations depends on the P\'eclet number, which is the relative importance of advection and diffusion. We apply our model to transition disks and our simulated observations show that this effect would be easily observable with ALMA and in principle allows to put constraints on the strength of turbulence and the local gas density.