Astronomy and Astrophysics, volume 284, 907-918 (1994/4-3)
Spectral analysis of the hottest known helium-rich white dwarf: KPD0005+5106.
WERNER K., HEBER U. and FLEMING T.
Abstract (from CDS):
We present a non-LTE model atmosphere analysis of the helium-rich white dwarf KPD0005+5106. The analysis is based on optical spectra obtained at the Calar Alto 3.5m telescope, ultraviolet observations retrieved from the Hubble Space Telescope data archive, and X-ray data recorded during the ROSAT all-sky survey. From the optical and ultraviolet HeII line spectrum we find that KPD0005+5106 is the hottest helium-rich white dwarf known (Teff=120000K, logg=7). The atmospheric parameters place the star within the domain of the PG1159 group, however, the metal abundances in KPD0005+5106 (C=-3.5, N=-4.0, O≤-4, logarithm of number ratio relative to helium) are significantly different from the C- and O-rich PG1159 objects. The hydrogen deficiency suggests that KPD0005+5106 is a late He-flash object. However, the C and N abundances contradict the simple interpretation that He-rich intershell matter (as a product of the CNO burning process) is exposed at the surface after removal of the H-rich envelope by mass-loss. Instead we propose that the metal abundances may result from the interplay of radiative levitation and gravitational settling. The presence of several ultrahigh-excitation features like the OVIII emission lines cannot be explained even with the high photospheric temperature encountered. Furthermore we show that the X-ray spectrum as well cannot be photospheric in origin but may be assigned to a hot corona around KPD0005+5106.