M ar 1 99 9 VORTICES IN PROTOPLANETARY DISKS

We use a high order accuracy spectral code to carry out two-dimensional time-dependent numerical simulations of vortices in accretion disks. In particular , we examine the stability and the life time of vortices in circumstellar disks around young stellar objects. The results show that cyclonic vortices dissipate quickly, while anticyclonic vortices can survive in the flow for hundreds of orbits. When more than one vortex is present, the anticyclonic vortices interact through vorticity waves and merge together to form larger vortices. The exponential decay time τ of anticyclonic vortices is of the order of 30 − 60 orbital periods for a viscosity parameter α ≈ 10 −4 (and it increases to τ ≈ 315 for α = 10 −5), which is sufficiently long to allow heavy dust particles to rapidly concentrate in the core of anticyclonic vortices in protoplanetary disks. This dust concentration increases the local density of centimeter-size grains, thereby favoring the formation of larger scale objects which are then capable of efficiently triggering a gravitational instability. The relatively long-lived vortices found in this work may therefore play a key role in the formation process of giant planets.