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We present an analysis of intermediate-dispersion spectra and photometricdata of the newly identified cool, polluted white dwarf NLTT19868. The spectraobtained with X-shooter on the Very Large Telescope (VLT)-Melipal show stronglines of calcium, and several lines of magnesium, aluminium and iron. We usethese spectra and the optical-to-near infrared spectral energy distribution toconstrain the atmospheric parameters of NLTT19868. Our analysis shows thatNLTT19868 is iron poor with respect to aluminium and calcium. A comparison withother cool, polluted white dwarfs shows that the Fe to Ca abundance ratio(Fe/Ca) varies by up to approximately two orders of magnitudes over a narrowtemperature range with NLTT19868 at one extremum in the Fe/Ca ratio and, incontrast, NLTT888 at the other extremum. The sample shows evidence of extremediversity in the composition of the accreted material: In the case of NLTT888,the inferred composition of the accreted matter is akin to iron-rich planetarycore composition, while in the case of NLTT19868 it is close to mantle orbulk-Earth composition depleted by subsequent chemical separation at the bottomof the convection zone.

Extreme abundance ratios in the polluted atmosphere of the cool white dwarf NLTT19868