Abstract

The strong surface gravity of white dwarfs causes metals to sink out of the atmosphere on time-scales much shorter than their cooling ages, leading to pristine H/He atmospheres. Therefore any metals detected in the atmosphere of a white dwarf imply recent or ongoing accretion of planetary debris. Determining the photospheric abundances of debris-polluted white dwarfs is hence analogue to the use of meteorites for measuring the abundances of planetary material in the solar system. I will review the results of a large, unbiased HST/COS survey of relatively young (~20-100Myr) white dwarfs. At least 27% of all white dwarfs in our sample are accreting planetary debris, and that fraction may be as high as 50%. The low C/Si ratio found for most polluted WDs confirms the rocky nature of the debris, though some white dwarfs are accreting icy material, and C/O ratios imply that ""carbon planets"" are not common. Up to 11 elements are detected in the most polluted stars, enabling a detailed comparison between the chemistry of exo-planetary material with that of solar system meteorites. We find a wide spread in the relative abundances of Mg, Fe, Si, and O, a constant Al/Ca ratio, and evidence for differentiation of Fe & Ni. The median progenitor mass is ~2Msun, and about half of the WDs descending from late B and A-type stars are debris-polluted, demonstrating that the formation of rocky material around 2-3Msun stars is common.

Presentation

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