Unusual Abundances from Planetary System Material Polluting the White Dwarf G238-44

TL;DR

This study analyzes the atmospheric composition of white dwarf G238-44, revealing unusual elemental abundances and evidence of accretion from multiple planetary bodies, including rocky and icy objects, indicating a highly perturbed exoplanetary system.

Contribution

It presents the first detection of nitrogen in a white dwarf polluted by planetary material and models the accreted material as a mix of rocky and icy bodies, revealing complex planetary system dynamics.

Findings

Detection of ten heavy elements, including nitrogen, in G238-44's atmosphere.

Stable accretion over 24 years suggests a continuous circumstellar reservoir.

Evidence for a mix of iron-rich and icy planetary material.

Abstract

Ultraviolet and optical spectra of the hydrogen-dominated atmosphere white dwarf star G238-44 obtained with FUSE, Keck/HIRES, HST/COS, and HST/STIS reveal ten elements heavier than helium: C, N, O, Mg, Al, Si, P, S, Ca, and Fe. G238-44 is only the third white dwarf with nitrogen detected in its atmosphere from polluting planetary system material. Keck/HIRES data taken on eleven nights over 24 years show no evidence for variation in the equivalent width of measured absorption lines, suggesting stable and continuous accretion from a circumstellar reservoir. From measured abundances and limits on other elements we find an anomalous abundance pattern and evidence for the presence of metallic iron. If the pollution is from a single parent body, then it would have no known counterpart within the solar system. If we allow for two distinct parent bodies, then we can reproduce the observed abundances with a mix of iron-rich Mercury-like material and an analog of an icy Kuiper Belt object with a respective mass ratio of 1.7:1. Such compositionally disparate objects would provide chemical evidence for both rocky and icy bodies in an exoplanetary system and would be indicative of a planetary system so strongly perturbed that G238-44 is able to capture both asteroid- and Kuiper Belt-analog bodies near-simultaneously within its $<$100 Myr cooling age.