在已知最热白矮星（KPD0005 +5106）远紫外光谱中发现光球CaX发射线
Discovery of photospheric CaX emission lines in the far-UV spectrum of the hottest known white dwarf (KPD0005+5106)
摘要：
我们首次在白矮星的远紫外光谱中发现了光球发射谱线。它们是在热（Teff~ 200，000 K）DO白矮星KPD 0005 +5106的远紫外光谱探测器光谱中发现的，它们来自极高电离的钙（CaX 1137，1159 Ang）。它们的光球起源证实了非LTE线形成计算。这是有史以来在恒星光球中观察到的任何元素的最高电离阶段。在任何热的白矮星或行星状星云的中心恒星中从未探测到钙。KPD 0005 +5106（1-10倍太阳）的钙丰度测定是困难的，因为线强度对目前有效温度和表面重力知识的不确定性相当敏感。我们讨论的可能性，钙丰度远低于预期的扩散/悬浮平衡理论。[WCE]型中心恒星NGC 2371也显示出相同的发射线。另一个CaX线对（1461，1504 Ang）可能出现在哈勃太空望远镜的PG 1159型中心恒星NGC 246的光谱中。
英文摘要：
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 (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.
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