  There is considerable interest in hydrodynamic instabilities in dead zones of
protoplanetary disks as a mechanism for driving angular momentum transport and
as a source of particle-trapping vortices to mix chondrules and incubate
planetesimal formation. We present simulations with a pseudo-spectral anelastic
code and with the compressible code Athena, showing that stably stratified
flows in a shearing, rotating box are violently unstable and produce
space-filling, sustained turbulence dominated by large vortices with Rossby
numbers of order 0.2-0.3. This Zombie Vortex Instability (ZVI) is observed in
both codes and is triggered by Kolmogorov turbulence with Mach numbers less
than 0.01. It is a common view that if a given constant density flow is stable,
then stable vertical stratification should make the flow even more stable. Yet,
we show that sufficient vertical stratification can be unstable to ZVI. ZVI is
robust and requires no special tuning of boundary conditions, or initial radial
entropy or vortensity gradients (though we have studied ZVI only in the limit
of infinite cooling time). The resolution of this paradox is that stable
stratification allows for a new avenue to instability: baroclinic critical
layers. ZVI has not been seen in previous studies of flows in rotating,
shearing boxes because those calculations frequently lacked vertical density
stratification and/or sufficient numerical resolution. Although we do not
expect appreciable angular momentum transport from ZVI in the small domains in
this study, we hypothesize that ZVI in larger domains with compressible
equations may lead to angular transport via spiral density waves.
