Analysis of HST/COS spectra of the bare C-O stellar core H1504+65 and a high-velocity twin in the Galactic halo

TL;DR

This study analyzes ultraviolet spectra of the unique hot white dwarf H1504+65 and identifies a Galactic halo twin, revealing insights into their composition, origin, and high-velocity movement.

Contribution

It provides the first detailed UV spectral analysis of H1504+65 and identifies a high-temperature twin in the Galactic halo, clarifying their nature and location.

Findings

H1504+65 has a carbon-oxygen dominated atmosphere with no hydrogen or helium.

The twin star is even hotter at 250,000 K and located in the Galactic halo.

The halo star recedes at +220 km/s and is 9.2 kpc away.

Abstract

H1504+65 is an extremely hot white dwarf (effective temperature Teff = 200,000 K) with a carbon-oxygen dominated atmosphere devoid of hydrogen and helium. This atmospheric composition was hitherto unique among hot white dwarfs (WDs), and it could be related to recently detected cooler WDs with C or O dominated spectra. The origin of the H and He deficiency in H1504+65 is unclear. To further assess this problem, we performed ultraviolet spectroscopy with the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope (HST). In accordance with previous far-ultraviolet spectroscopy performed with the Far Ultraviolet Spectroscopic Explorer, the most prominent lines stem from C IV, O V-VI, and Ne VI-VIII. Archival HST/COS spectra are utilized to prove that the supersoft X-ray source RX J0439.8-6809 is, considering the exotic composition, a twin of H1504+65 that is even hotter (Teff = 250,000 K). In contrast to earlier claims, we find that the star is not located in the Large Magellanic Cloud but a foreground object in the Galactic halo at a distance of 9.2 kpc, 5.6 kpc below the Galactic plane, receding with vrad = +220 km/s.