Discovery of photospheric CaX emission lines in the far-UV spectrum of the hottest known white dwarf (KPD0005+5106)

TL;DR

This paper reports the first detection of photospheric CaX emission lines in the far-UV spectrum of the hottest known white dwarf, revealing the highest ionization stage observed in a stellar photosphere and providing new insights into stellar composition.

Contribution

It presents the first identification of photospheric CaX emission lines in a white dwarf, confirmed by non-LTE calculations, and discusses implications for calcium abundance and stellar physics.

Findings

Detection of CaX emission lines in the white dwarf KPD0005+5106.

Calcium abundance estimated at 1-10 times solar.

Presence of similar lines in related stellar objects.

Abstract

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 stem 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.