On the equations of warped disc dynamics

TL;DR

This paper derives a unified set of equations for warped disc dynamics in both viscous and inviscid regimes using the warped shearing box framework, clarifying the physics behind warp propagation and damping.

Contribution

It provides a derivation of the unified warped disc equations from first principles using the warped shearing box approach, bridging the gap between viscous and wave-like regimes.

Findings

Unified equations valid for small warps in both regimes.

Clarification of the physical interpretation of warp evolution.

Connection to the affine tilted-slab model.

Abstract

The 1-D evolution equations for warped discs come in two flavors: For very viscous discs the internal torque vector G is uniquely determined by the local conditions in the disc, and warps tend to damp out rapidly if they are not continuously driven. For very inviscid discs, on the other hand, G becomes a dynamic quantity, and a warp will propagate through the disc as a wave. The equations governing both regimes are usually treated separately. A unified set of equations was postulated recently by Martin et al. (2019), but not yet derived from the underlying physics. The standard method for deriving these equations is based on a perturbation series expansion, which is a powerful, but somewhat abstract technique. A more straightforward method is to employ the warped shearing box framework of Ogilvie and Latter (2013), which so far has not yet been used to derive the equations for the wavelike regime. The goal of this paper is to analyze the warped disc equations in both regimes using the warped shearing box framework, to derive a unified set of equations, valid for small warps, and to discuss how our results can be interpreted in terms of the affine tilted-slab approach of Ogilvie (2018).