An aluminum/calcium-rich, iron-poor, white dwarf star: evidence for an extrasolar planetary lithosphere?

TL;DR

This study analyzes the elemental composition of a white dwarf star, suggesting it hosts a differentiated rocky planet with a crust-mantle structure, possibly due to a collision that removed part of its outer layers.

Contribution

It provides evidence for an extrasolar planetary lithosphere by comparing white dwarf pollution patterns with terrestrial and planetary materials.

Findings

NLTT 43806 is enriched in aluminum and poor in iron.

The composition matches a mixture of crust and mantle material.

Indicates a collision event stripping planetary outer layers.

Abstract

The presence of elements heavier than helium in white dwarf atmospheres is often a signpost for the existence of rocky objects that currently or previously orbited these stars. We have measured the abundances of various elements in the hydrogen-atmosphere white dwarfs G149-28 and NLTT 43806. In comparison with other white dwarfs with atmospheres polluted by heavy elements, NLTT 43806 is substantially enriched in aluminum but relatively poor in iron. We compare the relative abundances of Al and eight other heavy elements seen in NLTT 43806 with the elemental composition of bulk Earth, with simulated extrasolar rocky planets, with solar system meteorites, with the atmospheric compositions of other polluted white dwarfs, and with the outer layers of the Moon and Earth. Best agreement is found with a model that involves accretion of a mixture of terrestrial crust and upper mantle material onto NLTT 43806. The implication is that NLTT 43806 is orbited by a differentiated rocky planet, perhaps quite similar to Earth, that has suffered a collision that stripped away some of its outer layers.