Chemical Abundances in the Externally Polluted White Dwarf GD 40: Evidence of a Rocky Extrasolar Minor Planet

TL;DR

This study analyzes the atmospheric composition of the white dwarf GD 40, revealing accretion of rocky, differentiated planetary material likely from a tidally disrupted minor planet, with implications for exoplanetary geology.

Contribution

First detailed elemental abundance analysis of GD 40's atmosphere linking it to a rocky exoplanetary body with evidence of differentiation.

Findings

Accreted material likely from a tidally disrupted minor planet.

Parent body composition similar to Earth's crust but with signs of differentiation.

Low water content and high-temperature processing inferred from elemental ratios.

Abstract

We present Keck/HIRES data with model atmosphere analysis of the helium-dominated polluted white dwarf GD 40, in which we measure atmospheric abundances relative to helium of 9 elements: H, O, Mg, Si, Ca, Ti, Cr, Mn, and Fe. Apart from hydrogen whose association with the other contaminants is uncertain, this material most likely accreted from GD 40's circumstellar dust disk whose existence is demonstrated by excess infrared emission. The data are best explained by accretion of rocky planetary material, in which heavy elements are largely contained within oxides, derived from a tidally disrupted minor planet at least the mass of Juno, and probably as massive as Vesta. The relatively low hydrogen abundance sets an upper limit of 10% water by mass in the inferred parent body, and the relatively high abundances of refractory elements, Ca and Ti, may indicate high-temperature processing. While the overall constitution of the parent body is similar to the bulk Earth being over 85% by mass composed of oxygen, magnesium, silicon and iron, we find n(Si)/n(Mg) = 0.30 +/- 0.11, significantly smaller than the ratio near unity for the bulk Earth, chondrites, the Sun, and nearby stars. This result suggests that differentiation occurred within the parent body.