Polluting white dwarfs with Oort cloud comets

Abstract

Observations point to old white dwarfs (WDs) accreting metals at a relatively constant rate over 8 Gyr. 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 characterize the dynamics of comets around a WD with a companion having semimajor axes on the orders of 10-100 au. We develop simulation techniques capable of integrating a large number (108) of objects over a 1 Gyr time-scale. 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 characterize 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 time-scale, (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 ∼108 g s-1, (iii) adding a planetary-mass companion reduces the pollution rate to ∼107 g s-1, and (iv) if the companion is stellar mass, with Mp 0.1 M·, the pollution rate reduces to ∼3 × 105 g 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.