Formation of planetary debris discs around white dwarfs II: Shrinking extremely eccentric collisionless rings

TL;DR

This paper investigates how white dwarf radiation influences the evolution of extremely eccentric debris rings, leading to orbit circularization and shrinking within the Roche radius, with implications for the formation of planetary debris discs.

Contribution

It provides a new analytical formula for the shrinking timescale of debris rings around white dwarfs, incorporating stellar and particle properties, advancing understanding of debris disc formation.

Findings

White dwarf radiation causes orbit circularization of debris rings.

The derived formula predicts shrinking timescales based on star and particle properties.

Debris disc sizes depend on white dwarf cooling age and particle size.

Abstract

The formation channel of the tens of compact debris discs which orbit white dwarfs (WDs) at a distance of one Solar radius remains unknown. Asteroids that survive the giant branch stellar phases beyond a few au are assumed to be dynamically thrust towards the WD and tidally disrupted within its Roche radius, generating extremely eccentric (e>0.98) rings. Here, we establish that WD radiation compresses and circularizes the orbits of super-micron to cm-sized ring constituents to entirely within the WD's Roche radius. We derive a closed algebraic formula which well-approximates the shrinking time as a function of WD cooling age, the physical properties of the star and the physical and orbital properties of the ring particles. The shrinking timescale increases with both particle size and cooling age, yielding age-dependent WD debris disc size distributions.