Numerical study of self-gravitating protoplanetary discs

TL;DR

This study investigates how self-gravity influences protoplanetary disc stability, showing that cooling prescriptions significantly affect gravitational instability development and turbulence strength in the inner disc regions.

Contribution

It introduces a variable cooling timescale model that demonstrates the conditions under which gravitational instability occurs in self-gravitating protoplanetary discs.

Findings

Constant cooling timescale prevents gravitational instability at small radii.

Temperature-dependent cooling promotes gravitational instability in inner regions.

Inner disc turbulence strength is found to be very low.

Abstract

In this paper, the effect of self-gravity on the protoplanetary discs is investigated. The mechanisms of angular momentum transport and energy dissipation are assumed to be the viscosity due to turbulence in the accretion disc. The energy equation is considered in situation that the released energy by viscosity dissipation is balanced with cooling processes. The viscosity is obtained by equality of dissipation and cooling functions, and is used for angular momentum equation. The cooling rate of the flow is calculated by a prescription, $d u/d t=-u/\tau_{cool}$, that $u$ and $\tau_{cool}$ are internal energy and cooling timescale, respectively. The ratio of local cooling to dynamical timescales $\Omega \tau_{cool}$ is assumed as a constant and also as a function of local temperature. The solutions for protoplanetary discs show that in situation of $\Omega \tau_{cool} = constant$, the disc does not show any gravitational instability in small radii for a typically mass accretion rate, $\dot{M} = 10^{-6} M_{\odot} yr^{-1}$, while by choosing $\Omega \tau_{cool}$ as a function of temperature, the gravitational instability for this amount of mass accretion rate or even less can occur in small radii. Also, by study of the viscous parameter $\alpha$, we find that the strength of turbulence in the inner part of self-gravitating protoplanetary discs is very low. These results are qualitatively consistent with direct numerical simulations of protoplanetary discs.