On fragmentation of turbulent self-gravitating discs in the long cooling time regime

TL;DR

This paper investigates whether driven turbulence in protoplanetary discs promotes gravitational fragmentation in the long cooling time regime, concluding that turbulence actually stabilizes discs rather than causing collapse.

Contribution

It provides numerical evidence that driven turbulence does not enhance fragmentation in long cooling time discs, challenging previous suggestions about turbulence-induced collapse.

Findings

Turbulent driving increases disc heating, stabilizing against gravitational instability.

High density regions from turbulent velocity kicks quickly dissipate, preventing collapse.

Sustaining supersonic turbulence in slowly cooling discs is very difficult.

Abstract

It has recently been suggested that in the presence of driven turbulence discs may be much less stable against gravitational collapse than their non turbulent analogs, due to stochastic density fluctuations in turbulent flows. This mode of fragmentation would be especially important for gas giant planet formation. Here we argue, however, that stochastic density fluctuations due to turbulence do not enhance gravitational instability and disc fragmentation in the long cooling time limit appropriate for planet forming discs. These fluctuations evolve adiabatically and dissipate away by decompression faster than they could collapse. We investigate these issues numerically in 2D via shearing box simulations with driven turbulence and also in 3D with a model of instantaneously applied turbulent velocity kicks. In the former setting turbulent driving leads to additional disc heating that tends to make discs more, rather than less, stable to gravitational instability. In the latter setting, the formation of high density regions due to convergent velocity kicks is found to be quickly followed by decompression, as expected. We therefore conclude that driven turbulence does not promote disc fragmentation in protoplanetary discs and instead tends to make the discs more stable. We also argue that sustaining supersonic turbulence is very difficult in discs that cool slowly.