Interpreting Brightness Asymmetries in Transition Disks: Vortex at Dead Zone or Planet Carved Gap Edges?

TL;DR

This study compares vortex formation at planet-carved gap edges and dead zone edges in protoplanetary disks using hydrodynamic simulations, revealing differences in vortex longevity, morphology, and observability with ALMA.

Contribution

It provides a detailed comparison of vortex characteristics in two formation scenarios, highlighting observable differences to distinguish their origins.

Findings

Vortices at dead zone edges are longer-lived than those at planetary gap edges.

Vortex size and shape differ significantly between the two scenarios.

ALMA observations can potentially identify vortex formation mechanisms based on brightness asymmetries.

Abstract

Recent sub-millimeter observations show non-axisymmetric brightness distributions with a horseshoe-like morphology for more than a dozen transition disks. The most accepted explanation for the observed asymmetries is the accumulation of dust in large-scale vortices. Protoplanetary disks vortices can form by the excitation of Rossby-wave instability in the vicinity of a steep pressure gradient, which can develop at the edges of a giant planet carved gap or at the edges of an accretionally inactive zone. We studied the formation and evolution of vortices formed in these two distinct scenarios by means of two-dimensional locally isothermal hydrodynamic simulations. We found that the vortex formed at the edge of a planetary gap is short-lived, unless the disk is nearly inviscid. In contrast, the vortex formed at the outer edge of a dead zone is long-lived. The vortex morphology can be significantly different in the two scenarios: the vortex radial and azimuthal extensions are ~1.5 and ~3.5 times larger for the dead zone edge compared to gap models. In some particular cases, the vortex aspect ratios can be similar in the two scenarios, however, the vortex azimuthal extensions can be used to distinguish the vortex formation mechanisms. We calculate predictions for vortex observability in the sub-millimeter continuum with ALMA. We found that the azimuthal and radial extent of brightness asymmetry correlates with vortex formation process, within the limitations of alpha-viscosity prescription.