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

摘要

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 (T-eff approximate to 200 000 K) DO white dwarf KPD0005+5106 and they stem from extremely highly ionized calcium (Ca x lambda lambda 1137, 1159 angstrom). 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 Ca x line pair(lambda lambda 1461, 1504 angstrom) is probably present in a Hubble Space Telescope spectrum of the PG1159-type central star NGC246.