Formation of planetary debris discs around white dwarfs I: Tidal disruption of an extremely eccentric asteroid

TL;DR

This paper investigates how highly eccentric asteroids are tidally disrupted near white dwarfs, forming eccentric debris rings, and emphasizes the role of additional forces in the evolution of planetary debris around white dwarfs.

Contribution

It provides an analytical and numerical study of tidal disruption of eccentric asteroids, revealing the formation of collisionless rings and the importance of external forces.

Findings

Disruption timescale depends on pericentre distance.

Asteroids form eccentric debris rings after disruption.

Additional forces are needed for debris to accrete onto the WD.

Abstract

25%-50% of all white dwarfs (WDs) host observable and dynamically active remnant planetary systems based on the presence of close-in circumstellar dust and gas and photospheric metal pollution. Currently-accepted theoretical explanations for the origin of this matter include asteroids that survive the star's giant branch evolution at au-scale distances and are subsequently perturbed onto WD-grazing orbits following stellar mass loss. In this work we investigate the tidal disruption of these highly-eccentric (e > 0.98) asteroids as they approach and tidally disrupt around the WD. We analytically compute the disruption timescale and compare the result with fully self-consistent numerical simulations of rubble piles by using the N-body code PKDGRAV. We find that this timescale is highly dependent on the orbit's pericentre and largely independent of its semimajor axis. We establish that spherical asteroids readily break up and form highly eccentric collisionless rings, which do not accrete onto the WD without additional forces such as radiation or sublimation. This finding highlights the critical importance of such forces in the physics of WD planetary systems.