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Polluted White Dwarfs: Constraints on the Origin and Geology of Exoplanetary Material

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White dwarfs that have accreted rocky planetary bodies provide unique insights regarding the bulk composition of exoplanetary material. The frequency of solar system-like chemistry and geology in the galaxy is poorly understood. In our work we use the observed pollutant chemical abundances to constrain where in the planetary system the pollutant bodies originated and their geological and collisional history. We find that at least 2, but possibly up to 12, of the 17 systems analysed in our work have accreted a body dominated by either a core-like or a mantle-like material. These bodies are expected to be fragments formed in collisions between larger bodies that have differentiated into a core and a mantle, providing evidence for differentiation in exosystems. The even spread in the core mass fraction of the pollutants, and the lack of crust-rich pollutants in the 17 systems studied here agree with such a collisional model. The compositions of many pollutants exhibit trends related to elemental volatility, which we link to the temperatures and, thus, the locations at which these bodies formed. Our analysis shows that the abundances observed in 13 of the 17 systems considered are consistent with the compositions of nearby stars in combination with a trend related to volatility. The even spread and large range in the predicted formation location of the pollutants suggests that pollutants arrive in white dwarf atmospheres with a roughly equal efficiency from a wide range of radial locations, from very close to the host star, where refractory species dominate, to outside of the nitrogen ice line, where many ice species are abundant.

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