2 00 6 Vortices in Thin , Compressible , Unmagnetized Disks

We consider the formation and evolution of vortices in a hydrodynamic shearing-sheet model. The evolution is done numerically using a version of the ZEUS code. Consistent with earlier results, an injected vorticity field evolves into a set of long-lived vortices each of which has radial extent comparable to the local scale height. But we also find that the resulting velocity field has positive shear stress Σδv r δv φ. This effect appears only at high resolution. The transport, which decays with time as t −1/2 , arises primarily because the vortices drive compressive motions. This result suggests a possible mechanism for angular momentum transport in low-ionization disks, with two important caveats: a mechanism must be found to inject vorticity into the disk, and the vortices must not decay rapidly due to three-dimensional instabilities.