Hubble Space Telescope ultraviolet spectroscopy of the hottest known helium-rich pre-white dwarf KPD 0005+5106

We present a model-atmosphere analysis of the ultraviolet echelle spectra of KPD 0005+5106 taken with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. The star is the hottest known pre-white dwarf (T_eff = 200 000 ± 20 000 K, log g= 6.7 ± 0.3 (cm/s²)). Its atmosphere is composed of helium with trace amounts of metals. It is of the so-called O(He) spectral type that comprises very hot helium-rich pre-white dwarfs whose origin is debated. From neon and silicon ionisation balances, we derive tighter constraints on the effective temperature (195 000 ± 15 000 K) and improve previous abundance determinations of these elements. We confirm the idea that KPD 0005+5106 is the descendant of an R Coronae Borealis (RCB) star, so is the outcome of a binary-white-dwarf merger. We discuss the relation of KPD 0005+5106 to other O(He) and RCB stars.