Unstable low-mass planetary systems as drivers of white dwarf pollution

摘要

At least 25% of white dwarfs show atmospheric pollution by metals, sometimesaccompanied by circumstellar dust or gas discs or (in the case of WD 1145+017)transiting disintegrating asteroids. Delivery of planetesimals to the whitedwarf by orbiting planets is a leading candidate to explain these phenomena.Here, we study systems of planets and planetesimals undergoing planet-planetscattering triggered by the star's post-main sequence mass loss, and testwhether this can maintain high rates of delivery over the several Gyr that theyare observed. We find that low-mass planets (Earth to Neptune mass) areefficient deliverers of material and can maintain the delivery for Gyr.Unstable low-mass planetary systems reproduce the observed delayed onset ofsignificant accretion, as well as the slow decay in accretion rates at latetimes. Higher-mass planets are less efficient, and the delivery only lasts arelatively brief time before the planetesimal populations are cleared. Theorbital inclinations of bodies as they cross the white dwarf's Roche limit areroughly isotropic, implying that significant collisional interactions ofasteroids, debris streams and discs can be expected. If planet-planetscattering is indeed responsible for the pollution of white dwarfs, many suchobjects, and their main-sequence progenitors, can be expected to host(currently undetectable) super-Earth planets on orbits of several au andbeyond.