Polluting White Dwarfs with Oort Cloud Comets

Abstract

Observations point to old white dwarfs (WDs) accreting metals at a relatively constant rate over 8~Gyrs. Exo-Oort clouds around WDs have been proposed as potential reservoirs of materials, with galactic tide as a mechanism to deliver distant comets to the WD's Roche limit. In this work, we characterise the dynamics of comets around a WD with a companion having semi-major axes on the orders of 10 - 100 AU. We develop simulation techniques capable of integrating a large number ($10^8$) of objects over a 1 Gyr timescale. Our simulations include galactic tide and are capable of resolving close-interactions with a massive companion. Through simulations, we study the accretion rate of exo-Oort cloud comets into a WD's Roche limit. We also characterise the dynamics of precession and scattering induced on a comet by a massive companion. We find that (i) WD pollution by an exo-Oort cloud can be sustained over a Gyr timescale, (ii) an exo-Oort cloud with structure like our own Solar System's is capable of delivering materials into an isolated WD with pollution rate $\sim 10^8\mathrm{g}{\mathrm{s}}^{-1}$, (iii) adding a planetary-mass companion reduces the pollution rate to $\sim 10^7\mathrm{g}{\mathrm{s}}^{-1}$, and (iv) if the companion is stellar-mass, with $M_p\gtrsim 0.1M_{\odot }$, the pollution rate reduces to $\sim 3\times 10^5\mathrm{g}{\mathrm{s}}^{-1}$ due to a combination of precession induced on a comet by the companion, a strong scattering barrier, and low-likelihood of direct collisions of comets with the companion.

arXiv comment19 content pages, 4 appendices, 18 figures; accepted for publication in MNRAS; doi: 10.1093/mnras/stae986

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