White dwarf pollution by hydrated planetary remnants: Hydrogen and Metals in WD J204713.76-125908.9

TL;DR

This study analyzes the polluted white dwarf WD J204713.76-125908.9, revealing accretion of water-rich planetary debris with a composition similar to Solar System chondrites, indicating water delivery to the star's planetary system.

Contribution

First detailed spectroscopic analysis of WD J204713.76-125908.9 showing accretion of volatile-rich debris with water content, advancing understanding of planetary system evolution around white dwarfs.

Findings

Detected nine metals in the star's atmosphere.

Parent body composition similar to Solar System carbonaceous chondrites.

Estimated water mass fraction of about 8%.

Abstract

WD J204713.76-125908.9 is a new addition to the small class of white dwarfs with helium-dominated photospheres that exhibit strong Balmer absorption lines and atmospheric metal pollution. The exceptional abundances of hydrogen observed in these stars may be the result of accretion of water-rich rocky bodies. We obtained far-ultraviolet and optical spectroscopy of WD J204713.76-125908.9 using the Cosmic Origin Spectrograph on-board the Hubble Space Telescope and X-shooter on the Very Large Telescope, and identify photospheric absorption lines of nine metals: C, O, Mg, Si, P, S, Ca, Fe and Ni. The abundance ratios are consistent with the steady state accretion of exo-planetesimal debris rich in the volatile elements carbon and oxygen, and the transitional element sulphur, by factors of seventeen, two, and four respectively compared to bulk Earth. The parent body has a composition akin to Solar System carbonaceous chondrites, and the inferred minimum mass, $1.6 \times 10^{20}$ g, is comparable to an asteroid 23 km in radius. We model the composition of the disrupted parent body, finding from our simulations a median water mass fraction of eight per cent.