Accreting White Dwarfs: An Unreview

TL;DR

This paper highlights unresolved questions in the physics of accreting white dwarfs, emphasizing the need for further observational, numerical, and theoretical research to understand disk accretion, winds, and precession.

Contribution

It identifies key open problems in AWD physics and proposes directions for future research to address these fundamental questions.

Findings

Uncertain mechanisms driving viscosity in neutral disks

Unknown origins of powerful winds and their feedback

Persistent retrograde precession and burst phenomena unexplained

Abstract

Accreting white dwarfs (AWDs) are among the best natural laboratories for understanding disk accretion. Their proximity, brightness, and purely classical nature make them ideal systems in which to probe the fundamental physics that governs the transport of angular momentum, the generation of outflows, and the coupling between disks, magnetospheres, and accretors. Yet despite decades of study, many critical questions remain unresolved. In this ``unreview'', we therefore focus not on what is known, but on what is unknown. What drives viscosity and sustains accretion in largely neutral disks? How are powerful winds launched, and how do they feed back on the disk and binary evolution? Why do so many systems show persistent retrograde precession, and what drives bursts in magnetic AWDs? By identifying these open problems -- and suggesting ways to resolve them -- we aim to motivate new observational, numerical, and theoretical efforts that will advance our understanding of accretion physics across all mass scales, from white dwarfs to black holes.