Accretion disc viscosity: what do warped discs tell us?

TL;DR

This paper examines observational evidence for viscosity in accretion discs, especially warped ones, highlighting discrepancies between theoretical models and observations, and suggesting current understanding of viscosity in quiescent discs is incomplete.

Contribution

It analyzes observational constraints on viscosity in non-planar accretion discs, comparing them with theoretical models and identifying significant gaps in current understanding.

Findings

Alignment timescales in ionized discs are shorter than observed.

Viscosity estimates in low-temperature discs are too high by two orders of magnitude.

Current models may underestimate the complexity of viscosity in quiescent discs.

Abstract

Standard, planar accretion discs operate through a dissipative mechanism, usually thought to be turbulent, and often modelled as a viscosity. This acts to take energy from the radial shear, enabling the flow of mass and angular momentum in the radial direction. In a previous paper we discussed observational evidence for the magnitude of this viscosity, and pointed out discrepancies between these values and those obtained in numerical simulations. In this paper we discuss the observational evidence for the magnitude of the dissipative effects which act in non--planar discs, both to transfer and to eliminate the non--planarity. Estimates based on the model by Ogilvie (1999), which assumes a small--scale, isotropic viscosity, give alignment timescales for fully ionized discs which are apparently too short by a factor of a few compared with observations, although we emphasize that more detailed computations as well as tighter observational constraints are required to verify this conclusion. For discs with low temperature and conductivity, we find that the timescales for disc alignment based on isotropic viscosity are too short by around two orders of magnitude. This large discrepancy suggests that our understanding of viscosity in quiescent discs is currently inadequate.