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

TL;DR

This study analyzes the atmospheric composition of the cool white dwarf NLTT19868, revealing extreme abundance ratios and diversity in accreted planetary material, with implications for understanding planetary debris around white dwarfs.

Contribution

It provides detailed spectroscopic analysis of NLTT19868, highlighting its unique chemical abundance ratios and comparing them with other white dwarfs to reveal diversity in accreted planetary debris.

Findings

NLTT19868 is iron-poor relative to aluminium and calcium.

Fe/Ca ratio varies by up to two orders of magnitude among similar white dwarfs.

Accreted material shows diversity, from iron-rich core-like to mantle-like compositions.

Abstract

We present an analysis of intermediate-dispersion spectra and photometric data of the newly identified cool, polluted white dwarf NLTT19868. The spectra obtained with X-shooter on the Very Large Telescope (VLT)-Melipal show strong lines of calcium, and several lines of magnesium, aluminium and iron. We use these spectra and the optical-to-near infrared spectral energy distribution to constrain the atmospheric parameters of NLTT19868. Our analysis shows that NLTT19868 is iron poor with respect to aluminium and calcium. A comparison with other 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 narrow temperature range with NLTT19868 at one extremum in the Fe/Ca ratio and, in contrast, NLTT888 at the other extremum. The sample shows evidence of extreme diversity in the composition of the accreted material: In the case of NLTT888, the inferred composition of the accreted matter is akin to iron-rich planetary core composition, while in the case of NLTT19868 it is close to mantle or bulk-Earth composition depleted by subsequent chemical separation at the bottom of the convection zone.