Discovery of photospheric Ca X emission lines in the far - UV spectrum of the hottest known white dwarf ( KPD 0005 + 5106 ) ⋆

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 KPD 0005+5106 and they stem from extremely highly ionized calcium (Ca X λλ 1137, 1159 Å). 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 KPD 0005+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 NGC 2371. Another Ca X line pair (λλ 1461, 1504 Å) is probably present in a Hubble Space Telescope spectrum of the PG1159-type central star NGC 246.