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

## TL;DR

This paper investigates 3D gravitoturbulence in astrophysical discs, revealing how small-scale parametric instabilities are excited by large-scale modes, with implications for disc evolution and planet formation.

## Contribution

It demonstrates the independence of gravitoturbulent properties from domain size beyond a certain scale and identifies a parametric instability involving inertial waves and epicyclic modes.

## Key findings

- Small-scale turbulence appears off the midplane at high resolution.
- Certain gravitoturbulent properties are size-independent for boxes larger than ~40 H_0.
- A parametric instability couples inertial waves with large-scale epicyclic modes.

## 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 $\simeq 40 H_0$, where $H_0$ is the height scale, (b) at high resolution, small-scale isotropic turbulence appears off the midplane around $z\simeq 0.5 -1 H_0$, 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.