The Discovery of the Most Metal-Rich White Dwarf: Composition of a Tidally Disrupted Extrasolar Dwarf Planet

TL;DR

This paper reports the discovery and analysis of the most metal-rich white dwarf, revealing accretion of material from a tidally disrupted extrasolar dwarf planet similar in composition to Bulk Earth.

Contribution

It presents the first detailed composition analysis of a white dwarf polluted by a disrupted extrasolar body, combining spectroscopic, photometric, and modeling data.

Findings

White dwarf shows the highest metal pollution observed.

Atmospheric composition matches that of Bulk Earth.

Evidence of a circumstellar disk from infrared excess.

Abstract

Cool white dwarf stars are usually found to have an outer atmosphere that is practically pure in hydrogen or helium. However, a small fraction have traces of heavy elements that must originate from the accretion of extrinsic material, most probably circumstellar matter. Upon examining thousands of Sloan Digital Sky Survey spectra, we discovered that the helium-atmosphere white dwarf SDSS J073842.56+183509.6 shows the most severe metal pollution ever seen in the outermost layers of such stars. We present here a quantitative analysis of this exciting star by combining high S/N follow-up spectroscopic and photometric observations with model atmospheres and evolutionary models. We determine the global structural properties of our target star, as well as the abundances of the most significant pollutants in its atmosphere, i.e., H, O, Na, Mg, Si, Ca, and Fe. The relative abundances of these elements imply that the source of the accreted material has a composition similar to that of Bulk Earth. We also report the signature of a circumstellar disk revealed through a large infrared excess in JHK photometry. Combined with our inferred estimate of the mass of the accreted material, this strongly suggests that we are witnessing the remains of a tidally disrupted extrasolar body that was as large as Ceres.