Gravito-turbulence and the excitation of small-scale parametric instability in astrophysical discs

Abstract

Young protoplanetary discs and the outer radii of active galactic nucleii may be subject to gravitational instability and, as a consequence, fall into a `gravitoturbulent' state. While in this state, appreciable angular momentum can be transported. Alternatively, the gas may collapse into bound clumps, the progenitors of planets or stars. In this paper, we numerically characterize the properties of 3D gravitoturbulence, focussing especially on its dependence on numerical parameters (resolution, domain size) and its excitation of small-scale dynamics. Via a survey of vertically stratified shearing box simulations with PLUTO and RODEO, we find (a) evidence that certain gravitoturbulent properties are independent of horizontal box size only when the box is larger than $\simeq 40H_0$, where $H_0$ is the height scale, (b) at high resolution, small-scale isotropic turbulence appears off the midplane around $z\simeq 0.5-1H_0$, and (c) this small-scale dynamics results from a parametric instability, involving the coupling of inertial waves with a large-scale axisymmetric epicyclic mode. This mode oscillates at a frequency close to $\Omega$ and is naturally excited by gravito-turbulence, via a nonlinear process to be determined. The small-scale turbulence we uncover has potential implications for a wide range of disc physics, e.g. turbulent saturation levels, fragmentation, turbulent mixing, and dust settling.

22 pages, 19 figures, accepted for publication in MNRAS

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• Magnetorotational instability and dynamo action in gravitoturbulent astrophysical discs
• Characterizing gravito-turbulence in 3D: turbulent properties and stability against fragmentation
• Spiral structures in gravito-turbulent gaseous disks
• Planetesimal Initial Mass Functions following Diffusion Regulated Gravitational Collapse
• Particle Dynamics in 3D Self-gravitating Disks II: Strong Gas Accretion and Thin Dust Disks
• Formation of pebbles in (gravito-)viscous protoplanetary disks with various turbulent strengths
• Gravito-turbulence in local disk simulations with an adaptive moving mesh
• MRI turbulence in vertically stratified accretion discs at large magnetic Prandtl numbers
• Gravito-turbulence and dynamo in poorly ionised protostellar discs. I. Zero-net-flux case
• Dust growth and planet formation by disc fragmentation
• Gravitoturbulent dynamo in global simulations of gaseous disks
• Nonlinear analysis of gravitational instability in a 3D gaseous disc