Planetary engulfment as a trigger for white dwarf pollution

TL;DR

This paper proposes that planetary engulfment during stellar evolution can trigger gravitational instabilities, leading to white dwarf pollution by rocky debris, explaining observed high-metallicity atmospheres.

Contribution

It introduces a new mechanism where planetary engulfment triggers secular instabilities via the Kozai-Lidov effect, causing white dwarf pollution.

Findings

Engulfment during the AGB phase can induce instabilities leading to debris accretion.

The mechanism explains up to nearly half of observed polluted white dwarfs.

Steady pollution can occur throughout white dwarf cooling ages.

Abstract

The presence of a planetary system can shield a planetesimal disk from the secular gravitational perturbations due to distant outer massive objects (planets or stellar companions). As the host star evolves off the main sequence to become a white dwarf, these planets can be engulfed, triggering secular instabilities and leading to the tidal disruptions of small rocky bodies. These disrupted bodies can feed the white dwarfs with rocky material and possibly explain the high-metallicity material in their atmospheres. We illustrate how this mechanism can operate when the gravitational perturbations are due to the Kozai-Lidov mechanism from a stellar binary companion. We show that this mechanism can explain the observed levels of accretion if: (1) the planetary engulfment happens fast compared to the secular timescale, which is generally the case for wide binaries ($>100$ AU) and planetary engulfment during the Asymptotic Giant Branch; (2) the planetesimal disk has a total mass of $\sim10^{-4}-10^{-2}M_\oplus$. We show that this new mechanism can provide a steady supply of material throughout the entire life of the white dwarfs for all cooling ages and can account for a large fraction (up to nearly half) of the observed polluted WDs.