Metal abundances in the hottest known DO white dwarf (KPD 0005+5106)

Abstract

We present the analysis of ultraviolet and optical spectra of KPD 0005+5106. Redetermination of the atmospheric parameters is necessary because the recent discovery of highly ionised metals (Ne viii, Ca x) indicates that the effective temperature is significantly higher than previously thought. Here we announce the discovery of lines from highly ionised silicon, sulphur, and iron (Si vii, S vii, Fe x) which were never found before in any stellar photosphere. Our analysis of these lines and those from helium and several other metals gives T eff = 200 000 ± 20 000 K, log g = 6.7 ± 0.3. Mass and luminosity follow from a comparison with evolutionary tracks: M = 0.64 M and log L/L = 3.7, so that, strictly speaking, the star is a helium-burning pre-white dwarf. The mass fractions of the metals in the helium-dominated atmosphere are in the range 0.7−4.3 solar. Hydrogen is not detectable and we derive an upper abundance limit of 0.034 solar. This abundance pattern is probably unaffected by gravitational settling and radiative levitation. Its origin lies in previous evolutionary stages. We discuss the link of KPD 0005+5106 to RCrB stars and the possibility that it results from a double-degenerate merger event.

HubbleSpace Telescope ultraviolet spectroscopy of the hottest known helium-rich pre-white dwarf KPD 0005+5106

“…Our analysis builds on the results of Wassermann et al (2010). We computed a basic model with their derived values for effective temperature (T eff = 200 000 K), gravity (log g = 6.7), and element abundances.…”

“…In particular, we checked whether we can constrain T eff better than in the previous work (±20 000 K). We do not improve the gravity determination of Wassermann et al (2010). It was based on a careful analysis of all He ii lines in the UV and optical bands, including λ1640 Å.…”

“…There is now general consensus that RCB stars have a binary WD merger origin (Webbink 1984), based on evidence from evolution timescales, pulsation masses, and surface element abundances (Saio & Jeffery 2002;Pandey et al 2006;Clayton et al 2007). Closely related are the Extreme Helium stars (EHe), which are hotter, early-type (A and B) supergiants with similar photospheric composition, and they are assumed descendants of the RCB stars (e.g., Jeffery et al 2011).…”

“…The abundance pattern of KPD 0005+5106 was derived by the analysis of optical but mainly far-ultraviolet spectra taken with the Far-Ultraviolet Spectroscopic Explorer (FUSE), covering the wavelength range 912−1180 Å Wassermann et al 2010). In this paper, we present new ultraviolet (UV) spectra taken with the Hubble Space Telescope (HST), stretching the observed spectral range up to the optical.…”

New hydrogen-deficient (pre-) white dwarfs in the Sloan Digital Sky Survey Data Release 10

“…In that case a hydrogen-rich (DA) white dwarf will emerge. In view of 1 We exclude KPD 0005+5106 which was classified as DO but actually is a pre-WD (T eff = 200 000 K, log g = 6.7; Wassermann et al 2010). the large number of known WDs, the number of PG 1159 stars and hot DOs is small.…”