UC Berkeley

One of the most important problems in astrophysics is how angular momentum is transported in protoplanetary disks (PPDs - disks containing gas and dust orbit around newly-forming protostars). Collisional viscosity is believed to be insufficient for angular momentum transport. Therefore, turbulence enhanced transport are proposed. In addition, long-lived coherent vortices are also speculated to exist in PPDs, which could play an important role in completing star formation and building planets. Without instabilities, turbulence and vortices cannot form. In weak magnetized PPDs, magneto-rotational instability (MRI) operates to generate turbulence. However, regions known as ``dead zone", are cool and unionized to have MRI. This has led to intense theoretical and computational search for pure hydrodynamic instabilities.

A new hydrodynamic, finite amplitude instability has been discovered in linearly stable, rotating, stably-stratified, shear flows. The instability starts from a new family of critical layers - baroclinic critical layers. These critical layers, which are linear, neutrally stable eigenmodes in stratified shear flows, have singularity in their vertical velocities. Under the effect of rotation, these critical layers produce vortex layers. Vortex layers intensify by drawing energy from the background shear flows, and subsequently roll up to create new vortices, which in turn excite new critical layers. The whole process self-replicates until the whole domain is filled with large-volume, large amplitude vortices. Because this instability can occur in the dead zones of protoplanetary disks we refer it as zombie instability and these new class of vortices that self-replicate as zombie vortices. High resolution numerical simulations show this instability can be triggered by a variety of weak perturbations including small volume compact single vortex, a pair of vortices and noise. The threshold of the instability is determined by the Rossby number or vorticity of the initial perturbations. Energy analysis based on the zonal non-zonal decomposition of the energy shows energy that supplies the instability is extracted from the zonal flows. Vortex is responsible for the energy extraction process. Instability saturates when the all the space are taken by zombie vortices. The separation distance between zombie vortices is approximately the distance from critical layers with lowest stream-wise wave number to the perturbations. The flows at late time are determined only by the background parameters not their initial perturbations. Zombie instability is also discussed in a broader picture to show the dead zones of PPDs are not dead. Our numerical simulation suggest although zombie instability is a finite-amplitude instability, due to the large Reynolds number of the disk flows, it is effectively a linear instability. How zombie instability might lead to sufficient angular momentum transport is also discussed. Finally, we speculate there might not be a laminar Keplerian disks at all. The disk flows are essentially turbulence from the collapsing of gas cloud with possibility turbulent flows filled with zombie vortices. A newly developed semi-analytic method for flows with strong background shear is also presented to be an alternative to widely used shearing sheet method in the astrophysical community. The semi-analytic method is used for simulating internal inertial-gravity waves in rotating, stratified flows with and without shear. The method can also be generalized to systems with linear forcing terms.