Turbulent viscosity by convection in accretion discs - a self-consistent approach

TL;DR

This paper investigates how convective turbulence contributes to viscosity in accretion discs, finding that convection enhances but does not fully account for viscosity, with MRI playing a role at higher accretion rates.

Contribution

It presents a self-consistent method to evaluate convective turbulence's contribution to viscosity in accretion discs, including effects of self-gravity and MRI.

Findings

Convection significantly adds to viscosity but is insufficient alone.

Differential rotation provides enough viscosity at low accretion rates.

MRI becomes important for viscosity at higher accretion rates.

Abstract

The source of viscosity in astrophysical accretion flows is still a hotly debated issue. We investigate the contribution of convective turbulence to the total viscosity in a self-consistent approach, where the strength of convection is determined from the vertical disc structure itself. Additional sources of viscosity are parametrized by a beta-viscosity prescription, which also allows an investigation of self-gravitating effects. In the context of accretion discs around stellar mass and intermediate mass black holes, we conclude that convection alone cannot account for the total viscosity in the disc, but significantly adds to it. For accretion rates up to 10% of the Eddington rate, we find that differential rotation provides a sufficiently large underlying viscosity. For higher accretion rates, further support is needed in the inner disc region, which can be provided by an MRI-induced viscosity. We briefly discuss the interplay of MRI, convection and differential rotation. We conduct a detailed parameter study of the effects of central masses and accretion rates on the disc models and find that the threshold value of the supporting viscosity is determined mostly by the Eddington ratio with only little influence from the central black hole mass.