Cool DZ white dwarfs II: Compositions and evolution of old remnant planetary systems

TL;DR

This study analyzes the compositions of metal-polluted cool white dwarfs to understand the long-term evolution of their planetary systems, revealing accretion of diverse planetary debris and the influence of binary companions.

Contribution

It provides the first detailed analysis of core-like planetary debris accreted by old white dwarfs and demonstrates the decline in accretion rates over billions of years.

Findings

White dwarfs have accreted debris from a range of planetary compositions.

The accretion rate decreases by about 1000 times over 6.5 Gyr.

Binary companions can significantly perturb planetary systems.

Abstract

In a previous study, we analysed the spectra of 230 cool ($T_\mathrm{eff}$ < 9000 K) white dwarfs exhibiting strong metal contamination, measuring abundances for Ca, Mg, Fe and in some cases Na, Cr, Ti, or Ni. Here we interpret these abundances in terms of the accretion of debris from extrasolar planetesimals, and infer parent body compositions ranging from crust-like (rich in Ca and Ti) to core-like (rich in Fe and Ni). In particular, two white dwarfs, SDSSJ0823+0546 and SDSSJ0741+3146, which show log[Fe/Ca] > 1.9 dex, and Fe to Ni ratios similar to the bulk Earth, have accreted by far the most core-like exoplanetesimals discovered to date. With cooling ages in the range 1-8 Gyr, these white dwarfs are among the oldest stellar remnants in the Milky Way, making it possible to probe the long-term evolution of their ancient planetary systems. From the decrease in maximum abundances as a function of cooling age, we find evidence that the arrival rate of material on to the white dwarfs decreases by 3 orders of magnitude over a $\simeq$6.5 Gyr span in white dwarf cooling ages, indicating that the mass-reservoirs of post-main sequence planetary systems are depleted on a $\simeq$1 Gyr e-folding time-scale. Finally, we find that two white dwarfs in our sample are members of wide binaries, and both exhibit atypically high abundances, thus providing strong evidence that distant binary companions can dynamically perturb white dwarf planetary systems.