A road-map to white dwarf pollution: Tidal disruption, eccentric grind-down, and dust accretion

TL;DR

This paper outlines a comprehensive pathway for white dwarf pollution involving tidal disruption, fragment collision grind-down, and dust accretion, linking physical processes to observable signatures like IR excesses.

Contribution

It presents a detailed, multi-stage model connecting asteroid disruption to dust accretion and observable IR signatures, enhancing understanding of white dwarf pollution mechanisms.

Findings

High-velocity collisions cause rapid dust production from large asteroid fragments.

Optically thin dust accretion by Poynting-Robertson drag explains IR excesses above 10^7 g/s.

Enhanced drag mechanisms may be needed for dust circularisation in some high-rate accreting systems.

Abstract

A significant fraction of white dwarfs show metal lines indicative of pollution with planetary material but the accretion process remains poorly understood. The main aim of this paper is to produce a road-map illustrating several potential routes for white dwarf pollution and to link these paths to observational outcomes. Our proposed main road begins with the tidal disruption of a scattered asteroid and the formation of a highly eccentric tidal disc with a wide range of fragment sizes. Accretion of these fragments by Poynting-Robertson (PR) drag alone is too slow to explain the observed rates. Instead, in the second stage, several processes including differential apsidal precession cause high-velocity collisions between the eccentric fragments. Large asteroids produce more fragments when they disrupt, causing rapid grind-down and generating short and intense bursts of dust production, whereas smaller asteroids grind down over longer periods of time. In the final stage, the collisionally produced dust circularises and accretes onto the white dwarf via drag forces. We show that optically thin dust accretion by PR drag produces large infrared (IR) excesses when the accretion rate exceeds 10^7 g/s. We hypothesise that around white dwarfs accreting at a high rate, but with no detected infrared excess, dust circularisation requires enhanced drag - for instance due to the presence of gas near the disc's pericentre.