White dwarf pollution by planets in stellar binaries

Abstract

Approximately 0.2 ± 0.2 of white dwarfs (WDs) show signs of pollution by metals, which is likely due to the accretion of tidally disrupted planetary material. Models invoking planet- planet interactions after WD formation generally cannot explain pollution at cooling times of several Gyr.We consider a scenario in which a planet is perturbed by Lidov-Kozai oscillations induced by a binary companion and exacerbated by stellar mass-loss, explaining pollution at long cooling times. Our computed accretion rates are consistent with observations assuming planetary masses between ~0.01 and 1MMars, although non-gravitational effects may already be important for masses ≲0.3MMars. The fraction of pollutedWDs in our simulations, ~0.05, is consistent with observations of WDs with intermediate cooling times between ~0.1 and 1 Gyr. For cooling times ≲0.1 Gyr and ≳1 Gyr, our scenario cannot explain the high observed pollution fractions of up to 0.7. Nevertheless, our results motivate searches for companions around polluted WDs.