Analysis of HST/COS spectra of the bare C–O stellar core H1504+65 and a high-velocity twin in the Galactic halo ⋆
Institute for Astronomy and Astrophysics, Kepler Center for Astro and
Particle Physics, Eberhard Karls Universität Tübingen,
Received: 27 August 2015
Accepted: 24 September 2015
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-1.
Key words: stars: abundances / stars: atmospheres / stars: evolution / stars: AGB and post-AGB / white dwarfs
© ESO, 2015