Identification of Ne lines in H - deficient ( pre - ) white dwarfs : a new tool to constrain the temperature of the hottest stars ⋆ ⋆ ⋆

For the first time, we have identified Ne  absorption lines in far-UV spectra of the hottest known (Teff ∼150 000 K) hydrogendeficient (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 Ne  features and not, as hitherto suspected, as ultrahigh ionised O  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 (O -, C -) must be revised. These lines can be entirely attributed to thermally excited species (Ne -, N , O ). Photospheric Ne  lines are also identified in the hottest known He-rich white dwarf (KPD 0005+5106), some of which were also attributed to O  previously. This is a surprise because it must be concluded that KPD 0005+5106 is much hotter (Teff ≈200 000 K) than hitherto assumed (Teff ≈120 000 K). This is confirmed by a re-assessment of the He  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.