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

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.