Silicate mineralogy and bulk composition of exoplanetary material in polluted white dwarfs

TL;DR

This study links the mineralogy and bulk composition of exoplanetary material in white dwarf systems, revealing correlations between silicate mineralogy and elemental ratios, and demonstrating the use of equilibrium chemistry models to predict mineralogy.

Contribution

It provides new spectroscopic observations and models that connect circumstellar dust mineralogy with planetary bulk composition, advancing understanding of exoplanetary material in white dwarfs.

Findings

All systems show silicate features consistent with olivine and pyroxene.

WD1150-153 accretes Bulk Earth-like material.

GD56 accretes core-rich material with high core mass fraction.

Abstract

White dwarf planetary systems uniquely link the bulk elemental composition of exoplanetary material to the mineralogy as photospheric abundances can be compared to circumstellar dust mineralogy. This study re-examines Spitzer/IRS spectra of eight white dwarfs with both circumstellar dust and photospheric metals. All systems show 10$\mu$m silicate emission features consistent with a mixture of olivine and pyroxene silicates, with varying dominance. New Hubble Space Telescope ultraviolet spectroscopic observations of two of these systems, GD56 and WD1150-153, reveal that both are accreting dry, rocky material. WD1150-153 is accreting material consistent with Bulk Earth, while GD56 is accreting core-rich material with an inferred core mass fraction of 0.59$^{+0.08}_{-0.09}$ (0.37$^{+0.08}_{-0.08}$ by mole). A comparison between the bulk elemental composition of the accreted planetary material and the dust mineralogy of the eight systems reveals a tentative correlation between the dominant silicate mineralogy and the Mg/Si ratio, indicating that the circumstellar and photospheric material are compositionally similar. This suggests that rapid and well-mixed accretion is occurring with minimal compositional alteration. Furthermore, new GGCHEM equilibrium chemistry models confirm that Mg-rich planetary material preferentially forms olivine-rich dust, highlighting the importance of equilibrium in planetary chemistry and that a host star or rock's Mg/Si can be used to predict whether its silicate mineralogy is olivine- or pyroxene-dominated, influencing its capacity to structurally store water, recycle key nutrients, and possibly habitability.