Two Extrasolar Asteroids with Low Volatile-Element Mass Fractions

TL;DR

This study analyzes ultraviolet and optical spectra of two white dwarfs to determine the composition of accreted extrasolar asteroids, revealing low volatile-element mass fractions and suggesting thermal processing influences their composition.

Contribution

It provides the first combined ultraviolet and optical spectral analysis of extrasolar asteroids, revealing their low volatile content and similarities to Earth's bulk composition.

Findings

Volatile elements C and S are significantly depleted in GD 40.

Refractory element mass fractions are similar to Earth's and chondrites.

Thermal processing likely affects asteroid compositions.

Abstract

Using ultraviolet spectra obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope, we extend our previous ground-based optical determinations of the composition of the extrasolar asteroids accreted onto two white dwarfs, GD 40 and G241-6. Combining optical and ultraviolet spectra of these stars with He-dominated atmospheres, 13 and 12 polluting elements are confidently detected in GD 40 and G241-6, respectively. For the material accreted onto GD 40, the volatile elements C and S are deficient by more than a factor of 10 and N by at least a factor of 5 compared to their mass fractions in primitive CI chondrites and approach what is inferred for bulk Earth. A similar pattern is found for G241-6 except that S is undepleted. We have also newly detected or placed meaningful upper limits for the amount of Cl, Al, P, Ni and Cu in the accreted matter. Extending results from optical studies, the mass fractions of refractory elements in the accreted parent bodies are similar to what is measured for bulk Earth and chondrites. Thermal processing, perhaps interior to a snow line, appears to be of central importance in determining the elemental compositions of these particular extrasolar asteroids.