  Recent observations of large-scale asymmetric features in protoplanetary
disks suggest that large-scale vortices exist in such disks. Massive planets
are known to be able to produce deep gaps in protoplanetary disks. The gap
edges could become hydrodynamically unstable to the Rossby wave/vortex
instability and form large-scale vortices. In this study we examine the long
term evolution of these vortices by carrying out high-resolution two
dimensional hydrodynamic simulations that last more than $10^4$ orbits
(measured at the planet's orbit). We find that the disk viscosity has a strong
influence on both the emergence and lifetime of vortices. In the outer disk
region where asymmetric features are observed, our simulation results suggest
that the disk viscous $\alpha$ needs to be low $\sim 10^{-5 }$ - $10^{-4}$ to
sustain vortices to thousands and up to $10^{4}$ orbits in certain cases. The
chance of finding a vortex feature in a disk then decreases with smaller planet
orbital radius. For $\alpha \sim 10^{-3}$ or larger, even planets with masses
of 5 Jupiter-masses will have difficulty either producing or sustaining
vortices. We have also studied the effects of different disk temperatures and
planet masses. We discuss the implications of our findings on current and
future protoplanetary disk observations.
