  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.
