The comet bombardment rate of solitary white dwarfs

We have studied the dynamical evolution of a comet reservoir located at distances between ~10³ and ~10⁵ AU (an “Oort cloud”) during the late stages of stellar evolution, specifically the asymptotic giant branch (AGB) and white dwarf (WD) stages. Using N-body simulations, we examine how weakly anisotropic stellar mass loss during the AGB stage affects the fraction of Oort cloud comets that remain on bound orbits and the distribution function of surviving comets. We find that a small, but significant, fraction of comets remains bound to the newborn WD across a realistic range of kick velocities and mass-loss timescales. During the WD stage, we analytically estimate the rate at which the Galactic tidal field causes surviving Oort cloud comets to bombard the central star, leading to atmospheric metal pollution. We also use N-body simulations to investigate to what extent surviving planets orbiting the WD act as a barrier to comet bombardment. Based on our results, we suggest that the low observed prevalence of debris with a comet-like composition among polluted WDs indicates a significant population of unseen surviving planets acting as barriers to incoming Oort cloud comets. This is consistent with the inference of surviving planets in the ~10–100 AU range based on the observed accretion of debris from tidally disrupted rocky asteroids.