Search for technetium in extremely hot evolved stars

梅里尔（1952）在红巨星的大气中发现了锝（Tc），这有力地证明了s-过程核合成确实发生在演化的恒星中。这是因为Tc没有稳定的同位素，合成的99 Tc的半衰期为21万年。在这里，我们建议在极热的（前）白色矮星中寻找Tc，以澄清这些天体中大部分氢缺乏的起源。在这种情况下，Tc的存在是最近的直接祖星核合成的一个突出指标。然而，使用Tc作为指标的主要障碍是缺乏高电离阶段的原子数据。红巨星中的Tc是通过光谱中的Tc I线检测到的，而我们预计热白色矮星的紫外光谱中会有Tc IV-VII线。这些离子的能级、谱线位置和振子强度都是未知的，但这些数据对于谱线识别和恒星中元素丰度的测定是必要的。本项目提案结合了实验和理论工作，以便通过电子束离子阱光谱法确定Tc IV-VII线，获得原子数据，并将这些数据用于非LTE模型大气模拟计算恒星光谱，最终可以与天基望远镜获得的紫外线观测进行比较。

The discovery of technetium (Tc) in the atmospheres of red giants by Merrill (1952) constituted convincing proof that s-process nucleosynthesis is indeed occurring in evolved stars. This is because Tc has no stable isotopes, with the synthesized 99Tc having a half-life of 210,000 years. Here we propose to search for Tc in extremely hot (pre-) white dwarfs in order to clarify the origin of hydrogen-deficiency in a large fraction of these objects. In this context, the presence of Tc is an outstanding indicator for recent nucleosynthesis in the immediate progenitor stars. However, the main obstacle for using Tc as an indicator is the lack of atomic data for high ionization stages. Whereas Tc in red giants is detected by lines of Tc I in the optical spectra, we expect Tc IV-VII lines in the ultraviolet spectra of hot white dwarfs. Neither energy levels, nor line positions and oscillator strengths of these ions are known, but these data are necessary for line identification and element abundance determination in stars. The present project proposal combines experimental and theoretical work in order to identify Tc IV-VII lines by electron-beam ion trap spectroscopy, to derive atomic data, and to implement these data in non-LTE model-atmosphere simulations computing stellar spectra that can finally be compared to ultraviolet observations obtained with space-based telescopes.