March 17, 2015
Stewart Library,
2 p.m. |
Philip Marcus (University of California - Berkeley)
The Dead Zones in Protoplanetary Disks are Not Dead: They are Filled
with Self-Replicating Zombies that Bring Life to Stars and Planets
We show that there is a new purely hydrodynamic violent instability
of protoplanetary disks that destabilizes them, filling them with
turbulence. Our new instability is not a linear instability but requires
a finite perturbation. However, in a large Reynolds number flow (as
in a protoplanetary disk) with weak initial noise, the Mach number
of the initial noise can be very small – 10-7 – and still
destabilize the disk, completely filling it with strong vortices.
The energy of these vortices is supplied by the kinetic energy of
the background shear flow. The essential ingredients of the new instability
in our studies of rotating, shearing flows are the disk’s vertical
density stratification and vertical gravity, which are often ignored
in astrophysical calculations. The new instability is best analyzed
using the simplest possible initial perturbation – one isolated
vortex – rather than space-filling noise. In this simplest case,
the initial vortex triggers a new generation of vortices to grow at
nearby locations. After this second generation of vortices grows large,
it triggers a third generation. The triggering of subsequent generations
continues ad infinitum in a self-similar manner creating a 3D lattice
of turbulent 3D vortices. The region in protoplanetary disks where
we have found this new mechanism is thought to be stable; thus, in
the astrophysical literature this region is called the dead zone.
Because the vortices we report here arise in the dead zone, grow large,
and spawn new generations of vortices that march across the domain,
we refer to them as zombie vortices.
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