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

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 107 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.

The Geometry of the G29-38 White Dwarf Dust Disk from Radiative Transfer Modeling

“…Our model assumes an axisymmetric disk with a grain size distribution independent of location. Theoretical work suggests that disks around white dwarfs can be eccentric, especially during their initial formation from a tidally disrupted asteroid (Nixon et al 2020;Brouwers et al 2022). While our results show that the IR excess can be reproduced with a circular disk, they do not rule out disks with inherently different geometries.…”

“…The dust likely originates from planetesimals scattered into highly eccentric orbits then tidally disrupted upon close approach to the white dwarf star (Debes & Sigurdsson 2002;Jura 2003;Veras et al 2014). Actively disintegrating planetesimals have been seen via their transit signatures in a growing number of systems (Vanderburg et al 2015;Vanderbosch et al 2020Vanderbosch et al , 2021Guidry et al 2021;Farihi et al 2022) although the process by which the resulting debris forms a dust disk remains an active area of investigation (e.g., Veras et al 2015;Brouwers et al 2022).…”

“…Radiation pressure is insignificant for grains on circular orbits, as we assume for our model. Grains on very eccentric orbits around a white dwarf are susceptible to radiation pressure(Brouwers et al 2022). …”

WD 1145+017: Alternative models of the atmosphere, dust clouds, and gas rings

“…The abundance ratios of heavy elements in the atmospheres of some white dwarfs were found to closely match the abundance patterns in rocky bodies in the Solar System (Zuckerman et al 2007). A more detailed road map towards polluted white dwarfs with an IR excess was proposed recently by Brouwers et al (2022), which begins with the tidal disruption of an asteroid, followed by the formation of a highly eccentric tidal disk, and then collisional grind-down of fragments and their subsequent circularization and accretion.…”

Magnetic Interactions in Orbital Dynamics

“…host on dynamical timescales. Still, other influences, including interactions with a gas disk and collisions, may compete with or overwhelm the magnetic phenomenon described here (Brouwers et al 2022).…”