Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts

TL;DR

This study detects alkali metals in white dwarf atmospheres, revealing accreted planetary crust fragments and suggesting rocky planet formation around massive B-type stars, with implications for planetary differentiation and composition.

Contribution

It provides the first evidence of planetary crust material in white dwarf atmospheres using alkali metal abundances, expanding understanding of exoplanetary composition and formation.

Findings

Detection of lithium and potassium indicating planetary crusts

Infrared excess suggests ongoing debris disk accretion

Progenitor star mass implies formation around massive stars

Abstract

White dwarfs that accrete the debris of tidally disrupted asteroids provide the opportunity to measure the bulk composition of the building blocks, or fragments, of exoplanets. This technique has established a diversity in compositions comparable to what is observed in the solar system, suggesting that the formation of rocky planets is a generic process. Whereas the relative abundances of lithophile and siderophile elements within the planetary debris can be used to investigate whether exoplanets undergo differentiation, the composition studies carried out so far lack unambiguous tracers of planetary crusts. Here we report the detection of lithium in the atmospheres of four cool (<5,000 K) and old (cooling ages 5-10 Gyr) metal-polluted white dwarfs, where one also displays photospheric potassium. The relative abundances of these two elements with respect to sodium and calcium strongly suggest that all four white dwarfs have accreted fragments of planetary crusts. We detect an infrared excess in one of the systems, indicating that accretion from a circumstellar debris disk is on-going. The main-sequence progenitor mass of this star was $4.8\pm0.2 M_\odot$, demonstrating that rocky, differentiated planets may form around short-lived B-type stars.