A wide binary trigger for white dwarf pollution

TL;DR

This paper proposes a novel, age-independent mechanism involving wide binary companions perturbed by Galactic tides to explain metal pollution in white dwarf atmospheres, potentially accounting for pollution in older white dwarfs.

Contribution

It introduces a new mechanism where Galactic tide-perturbed wide binaries can cause white dwarf pollution, independent of the star's age, expanding current understanding.

Findings

Mechanism can contribute to pollution in up to a few percent of white dwarfs with wide companions.

Pollution occurs through perturbation of planetary systems by binary companions approaching close after billions of years.

Mechanism is independent of white dwarf cooling age, unlike previous models.

Abstract

Metal pollution in white dwarf atmospheres is likely to be a signature of remnant planetary systems. Most explanations for this pollution predict a sharp decrease in the number of polluted systems with white dwarf cooling age. Observations do not confirm this trend, and metal pollution in old (1-5 Gyr) white dwarfs is difficult to explain. We propose an alternative, time-independent mechanism to produce the white dwarf pollution. The orbit of a wide binary companion can be perturbed by Galactic tides, approaching close to the primary star for the first time after billions of years of evolution on the white dwarf branch. We show that such a close approach perturbs a planetary system orbiting the white dwarf, scattering planetesimals onto star-grazing orbits, in a manner that could pollute the white dwarf's atmosphere. Our estimates find that this mechanism is likely to contribute to metal pollution, alongside other mechanisms, in up to a few percent of an observed sample of white dwarfs with wide binary companions, independent of white dwarf age. This age independence is the key difference between this wide binary mechanism and others mechanisms suggested in the literature to explain white dwarf pollution. Current observational samples are not large enough to assess whether this mechanism makes a significant contribution to the population of polluted white dwarfs, for which better constraints on the wide binary population are required, such as those that will be obtained in the near future with Gaia.