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We present a detailed study of the metal-polluted DB white dwarf SDSSJ0845+2257 (Ton 345). Using high-resolution HST/COS and VLT spectroscopy, wehave detected hydrogen and eleven metals in the atmosphere of the white dwarf.The origin of these metals is almost certainly the circumstellar disc of dustyand gaseous debris from a tidally-disrupted planetesimal, accreting at a rateof 1.6E10 gs^-1. Studying the chemical abundances of the accreted materialdemonstrates that the planetesimal had a composition similar to the Earth,dominated by rocky silicates and metallic iron, with a low water content. Themass of metals within the convection zone of the white dwarf corresponds to anasteroid of at least ~130-170 km in diameter, although the presence of ongoingaccretion from the debris disc implies that the planetesimal was probablylarger than this. While a previous abundance study of the accreted material hasshown an anomalously high mass fraction of carbon (15 percent) compared to thebulk Earth, our independent analysis results in a carbon abundance of just 2.5percent. Enhanced abundances of core material (Fe, Ni) suggest that theaccreted object may have lost a portion of its mantle, possibly due to stellarwind stripping in the asymptotic giant branch. Time-series spectroscopy revealsvariable emission from the orbiting gaseous disc, demonstrating that theevolved planetary system at SDSS J0845+2257 is dynamically active.

The Composition Of A Disrupted Extrasolar Planetesimal At SDSS J0845+2257 (Ton 345)