The discovery of photospheric nickel in the hot DO white dwarf REJ 0503-289

We present the first evidence for the direct detection of nickel in the photosphere of the hot DO white dwarf REJ 0503-289. While this element has been seen previously in the atmospheres of hot H-rich white dwarfs, this is one of the first similar discoveries in a Herich object. Intriguingly, iron, which is observed to be more abundant than Ni in the hot DA stars, is not detected, the upper limit to its abundance (Fe/He = 10^-6) implying an Fe/Ni ratio a factor of 10 lower than seen in the H-rich objects (Ni/He = 10^-5 for REJ 0503-289). The abundances of nickel and various other elements heavier than He were determined from Goddard High Resolution Spectrograph spectra. We used two completely independent sets of non-local thermodynamic equilibrium model atmospheres, which both provide the same results. This not only reduces the possibility of systematic errors in our analysis, but is also an important consistency check for both model atmosphere codes. We have also developed a more objective method of determining T_eff and log g, from the He lines in the optical spectrum, in the form of a formal fitting of the line profiles to a grid of model spectra, an analogue of the standard procedure utilizing the Balmer lines in DA white dwarfs. This gives the assigned uncertainties in T_eff and log g a firm statistical basis and allows us to demonstrate that inclusion of elements heavier than H, He and C in the spectral calculations, exclusively considered in most published optical analyses, yields a systematic downward shift in the measured value of T_eff.