For the first time, we have identified photospheric emission lines in the far-UV spectrum of a white dwarf. They were discovered in the Far Ultraviolet Spectroscopic Explorer spectrum of the hot (Teff~200,000 K) DO white dwarf KPD0005+5106 and they s
tem from extremely highly ionized calcium (CaX 1137, 1159 Ang). Their photospheric origin is confirmed by non-LTE line-formation calculations. This is the highest ionisation stage of any element ever observed in a stellar photosphere. Calcium has never been detected before in any hot white dwarf or central star of planetary nebula. The calcium abundance determination for KPD0005+5106 (1-10 times solar) is difficult, because the line strengths are rather sensitive to current uncertainties in the knowledge of effective temperature and surface gravity. We discuss the possibility that the calcium abundance is much lower than expected from diffusion/levitation equilibrium theory. The same emission lines are exhibited by the [WCE]-type central star NGC2371. Another CaX line pair (1461, 1504 Ang) is probably present in a Hubble Space Telescope spectrum of the PG1159-type central star NGC246.
We report on our attempt for the first non-LTE modeling of gaseous metal disks around single DAZ white dwarfs recently discovered by Gaensicke et al. and thought to originate from a disrupted asteroid. We assume a Keplerian rotating viscous disk ring
composed of calcium and hydrogen and compute the detailed vertical structure and emergent spectrum. We find that the observed infrared CaII emission triplet can be modeled with a hydrogen-deficient gas ring located at R=1.2 R_sun, inside of the tidal disruption radius, with Teff about 6000 K and a low surface mass density of about 0.3 g/cm**2. A disk having this density and reaching from the central white dwarf out to R=1.2 R_sun would have a total mass of 7 10**21 g, corresponding to an asteroid with about 160 km diameter.
A possible origin of the iron-deficiency in PG1159 stars could be neutron captures on Fe nuclei. A nickel overabundance would corroborate this idea. Consequently we are looking for nickel lines in PG1159 stars. Prime targets are relatively cool objec
ts, because Ni VI is the dominant ionisation stage and the spectral lines of this ion are accessible with UV observations. We do not find such lines in the coolest PG1159 star observed by FUSE (PG1707+427, Teff = 85,000 K) and conclude that the nickel abundance is not enhanced. Hence, the Fe-deficiency in PG1159 stars remains unexplained. In addition, we present results of a wind analysis of the hybrid-PG1159 star NGC 7094 and the [WC]-PG1159 transition-type object Abell 78 in order to derive F abundances from the F VI 1139.5 Angstrom line. In both cases, we find F overabundances, in agreement with results of photospheric analyses of many PG1159 stars. Surprisingly, we find indications for a very low O abundance in NGC 7094.
The World Space Observatory Ultraviolet (WSO/UV) is a multi-national project grown out of the needs of the astronomical community to have future access to the UV range. WSO/UV consists of a single UV telescope with a primary mirror of 1.7m diameter f
eeding the UV spectrometer and UV imagers. The spectrometer comprises three different spectrographs, two high-resolution echelle spectrographs (the High-Resolution Double-Echelle Spectrograph, HIRDES) and a low-dispersion long-slit instrument. Within HIRDES the 102-310nm spectral band is split to feed two echelle spectrographs covering the UV range 174-310nm and the vacuum-UV range 102-176nm with high spectral resolution (R>50,000). The technical concept is based on the heritage of two previous ORFEUS SPAS missions. The phase-B1 development activities are described in this paper considering performance aspects, design drivers, related trade-offs (mechanical concepts, material selection etc.) and a critical functional and environmental test verification approach. The current state of other WSO/UV scientific instruments (imagers) is also described.
KPD0005+5106 is the hottest known helium-rich white dwarf. We have identified NeVIII lines in UV and optical spectra and conclude that it is significantly hotter than previously thought, namely Teff=200,000 K instead of 120,000 K. This is a possible
explanation for the observed hard X-ray emission as being of photospheric origin. Concerning its evolutionary state, we suggest that KPD0005+5106 is not a descendant of a PG1159 star but more probably related to the O(He) stars and RCrB stars.
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. T
hus, the photospheric elemental abundances of these stars allow to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted elemental abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative agreement is found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for others (P, S, Fe) point at shortcomings in stellar evolution models for AGB stars. PG1159 stars appear to be the direct progeny of [WC] stars.
For the first time, we have identified NeVIII absorption lines in far-UV spectra of the hottest known (Teff>150,000 K) hydrogen-deficient (pre-) white dwarfs of spectral type PG1159. They are of photospheric origin and can be matched by synthetic non
-LTE line profiles. We also show that a number of UV and optical emission lines in these stars can be explained as being photospheric NeVIII features and not, as hitherto suspected, as ultrahigh ionised OVIII lines created along shock-zones in the stellar wind. Consequently, we argue that the long-standing identification of the same emission lines in hot [WR]-type central stars as being due to ultrahigh-ionised species (OVII-VIII, CV-VI) must be revised. These lines can be entirely attributed to thermally excited species (NeVII-VIII, NV, OVI). Photospheric NeVIII lines are also identified in the hottest known He-rich white dwarf KPD0005+5106 some of which were also attributed to OVIII previously. This is a surprise because it must be concluded that KPD0005+5106 is much hotter (Teff=200,000 K) than hitherto assumed (Teff=120,000 K). This is confirmed by a re-assessment of the HeII line spectrum. We speculate that the temperature is high enough to explain the mysterious, hard X-ray emission (1 keV) as being of photospheric origin.
