Spiral structures in gravito-turbulent gaseous disks
  Gravitational instabilities can drive small-scale turbulence and large-scale
spiral arms in massive gaseous disks under conditions of slow radiative
cooling. These motions affect the observed disk morphology, its mass accretion
rate and variability, and could control the process of planet formation via
dust grain concentration, processing, and collisional fragmentation. We study
gravito-turbulence and its associated spiral structure in thin gaseous disks
subject to a prescribed cooling law. We characterize the morphology, coherence,
and propagation of the spirals and examine when the flow deviates from viscous
disk models. We used the finite-volume code Pluto to integrate the equations of
self-gravitating hydrodynamics in three-dimensional spherical geometry. The gas
was cooled over longer-than-orbital timescales to trigger the gravitational
instability and sustain turbulence. We ran models for various disk masses and
cooling rates. In all cases considered, the turbulent gravitational stress
transports angular momentum outward at a rate compatible with viscous disk
theory. The dissipation of orbital energy happens via shocks in spiral density
wakes, heating the disk back to a marginally stable thermal equilibrium. These
wakes drive vertical motions and contribute to mix material from the disk with
its corona. They are formed and destroyed intermittently, and they nearly
corotate with the gas at every radius. As a consequence, large-scale spiral
arms exhibit no long-term global coherence, and energy thermalization is an
essentially local process. In the absence of radial substructures or tidal
forcing, and provided a local cooling law, gravito-turbulence reduces to a
local phenomenon in thin gaseous disks.
