Numerical convergence in self-gravitating shearing sheet simulations and the stochastic nature of disc fragmentation

TL;DR

This study investigates the numerical convergence of 2D self-gravitating disc simulations, revealing stochastic fragmentation processes influenced by resolution and cooling, with implications for understanding disc stability.

Contribution

It demonstrates that disc fragmentation is a stochastic process affected by resolution and cooling, challenging previous convergence assumptions in 2D simulations.

Findings

Fragmentation occurs at longer cooling times than previously thought.

High resolution and long simulations are essential to capture stochastic clump formation.

2D simulations may not fully represent 3D disc dynamics.

Abstract

We study numerical convergence in local two-dimensional hydrodynamical simulations of self-gravitating accretion discs with a simple cooling law. It is well-known that there exists a steady gravito-turbulent state, in which cooling is balanced by dissipation of weak shocks, with a net outward transport of angular momentum. Previous results indicated that if cooling is too fast (typical time scale 3/Omega, where Omega is the local angular velocity), this steady state can not be maintained and the disc will fragment into gravitationally bound clumps. We show that, in the two-dimensional local approximation, this result is in fact not converged with respect to numerical resolution and longer time integration. Irrespective of the cooling time scale, gravito-turbulence consists of density waves as well as transient clumps. These clumps will contract because of the imposed cooling, and collapse into bound objects if they can survive for long enough. Since heating by shocks is very local, the destruction of clumps is a stochastic process. High numerical resolution and long integration times are needed to capture this behaviour. We have observed fragmentation for cooling times up to 20/Omega, almost a factor 7 higher than in previous simulations. Fully three-dimensional simulations with a more realistic cooling prescription are necessary to determine the effects of the use of the two-dimensional approximation and a simple cooling law.