Institute of Astronomy

Rocky Planetary Debris around young WDs

SpeakerTalk DateTalk Series
Boris Gaensicke31 July 2014Across HR 2014 Talks


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.