Search for technetium in extremely hot evolved stars

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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.