Two Beyond-Primitive Extrasolar Planetesimals

TL;DR

This study uses ultraviolet observations of polluted white dwarfs to analyze the composition of accreted planetesimals, revealing evidence of evolved, beyond-primitive bodies with diverse compositions similar to meteorites.

Contribution

It provides new elemental abundance measurements and demonstrates that accreted bodies are often beyond-primitive, with compositions akin to meteorites, based on high-resolution UV data.

Findings

GD 362's planetesimal similar to mesosiderite meteorite

PG 1225-079's planetesimal requires a blend of ureilite and mesosiderite

Evidence for both primitive and evolved planetesimals in white dwarf systems

Abstract

Using the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we have obtained high-resolution ultraviolet observations of GD 362 and PG 1225-079, two helium-dominated, externally-polluted white dwarfs. We determined or placed useful upper limits on the abundances of two key volatile elements, carbon and sulfur, in both stars; we also constrained the zinc abundance in PG 1225-079. In combination with previous optical data, we find strong evidence that each of these two white dwarfs has accreted a parent body that has evolved beyond primitive nebular condensation. The planetesimal accreted onto GD 362 had a bulk composition roughly similar to that of a mesosiderite meteorite based on a reduced chi-squared comparison with solar system objects; however, additional material is required to fully reproduce the observed mid-infrared spectrum for GD 362. No single meteorite can reproduce the unique abundance pattern observed in PG 1225-079; the best fit model requires a blend of ureilite and mesosiderite material. From a compiled sample of 9 well-studied polluted white dwarfs, we find evidence for both primitive planetesimals, which are a direct product from nebular condensation, as well as beyond-primitive planetesimals, whose final compositions were mainly determined by post-nebular processing.