Solar Abundances of Rock Forming Elements, Extreme Oxygen and Hydrogen in a Young Polluted White Dwarf

TL;DR

This study reveals that the white dwarf WD 1536+520 has solar-like abundances of rock-forming elements and an unusually high hydrogen level, indicating accretion of a water-rich rocky body and providing insights into planetary debris composition.

Contribution

It provides detailed elemental abundances of a white dwarf with the highest metal levels observed, suggesting accretion of a water-rich rocky parent body and analyzing diffusion timescales.

Findings

White dwarf shows solar-like rock-forming element abundances.

High hydrogen abundance indicates water-rich parent body accretion.

Metal and hydrogen signatures will fade within 300 million years.

Abstract

The Teff = 20,800 K white dwarf WD 1536+520 is shown to have broadly solar abundances of the major rock forming elements O, Mg, Al, Si, Ca, and Fe, together with a strong relative depletion in the volatile elements C and S. In addition to the highest metal abundances observed to date, including log(O/He) = -3.4, the helium-dominated atmosphere has an exceptional hydrogen abundance at log(H/He) = -1.7. Within the uncertainties, the metal-to-metal ratios are consistent with the accretion of an H2O-rich and rocky parent body, an interpretation supported by the anomalously high trace hydrogen. The mixed atmosphere yields unusually short diffusion timescales for a helium atmosphere white dwarf, of no more than a few hundred yr, and equivalent to those in a much cooler, hydrogen-rich star. The overall heavy element abundances of the disrupted parent body deviate modestly from a bulk Earth pattern, and suggest the deposition of some core-like material. The total inferred accretion rate is 4.2e9 g/s, and at least 4 times higher than any white dwarf with a comparable diffusion timescale. Notably, when accretion is exhausted in this system, both metals and hydrogen will become undetectable within roughly 300 Myr, thus supporting a scenario where the trace hydrogen is related to the ongoing accretion of planetary debris.