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# Alexander Mustill

Researcher

## Accretion of tidally disrupted asteroids on to white dwarfs : Direct accretion versus disc processing

Author

• Daohai Li
• Alexander J. Mustill
• Melvyn B. Davies

## Summary, in English

Atmospheric heavy elements have been observed in more than a quarter of white dwarfs (WDs) at different cooling ages, indicating ongoing accretion of asteroidal material, whilst only a few per cent of the WDs possess a dust disc, and all these WDs are accreting metals. Here, assuming that a rubble-pile asteroid is scattered inside a WD's Roche lobe by a planet, we study its tidal disruption and the long-Term evolution of the resulting fragments. We find that after a few pericentric passages, the asteroid is shredded into its constituent particles, forming a flat, thin ring. On a time-scale of Myr, tens of per cent of the particles are scattered on to the WD, and are therefore directly accreted without first passing through a circularized close-in disc. Fragment mutual collisions are most effective for coplanar fragments, and are thus only important in 103-104 yr before the orbital coplanarity is broken by the planet. We show that for a rubble pile asteroid with a size frequency distribution of the component particles following that of the near earth objects, it has to be roughly at least 10 km in radius such that enough fragments are generated and $\ge 10{{\ \rm per\ cent}}$ of its mass is lost to mutual collisions. At relative velocities of tens of km s-1, such collisions grind down the tidal fragments into smaller and smaller dust grains. The WD radiation forces may shrink those grains' orbits, forming a dust disc. Tidal disruption of a monolithic asteroid creates large km-size fragments, and only parent bodies ≥100 km are able to generate enough fragments for mutual collisions to be significant. Hence, those large asteroids experience a disc phase before being accreted.

Department/s

• Lund Observatory
• eSSENCE: The e-Science Collaboration

Publishing year

2021-12-01

Language

English

Pages

5671-5686

Publication/Series

Monthly Notices of the Royal Astronomical Society

Volume

508

Issue

4

Document type

Journal article

Publisher

Oxford University Press

Topic

• Astronomy, Astrophysics and Cosmology

Keywords

• accretion, accretion discs-planets and satellites: dynamical evolution and stability
• methods: numerical
• minor planets, asteroids: general
• white dwarfs

Status

Published

Project

• IMPACT: Comets, asteroids and the habitability of planets