0 Assigned ID: 1622 ----- Attempting Submission 1 (Thu Apr 08 14:13:20 GMT 2021) ----- The alpha elements magnesium (Mg), silicon (Si), and calcium (Ca) are of fundamental importance to a wide range of fields in astronomy, including stellar evolution, Galactic assembly, and planet formation. Questions about the reliability of their abundance measurements in cool stars remain, however. The proposed study will examine the Mg, Si, and Ca abundances from lines of both neutral and ionized species in a sample of 14 metal-poor stars to perform a new test of the reliability of current abundance methods. Mg, Si, and Ca abundances in stars are nearly always derived from lines of their neutral atoms, and the novel feature of the proposed study is in deriving their abundances based on lines of Mg+, Si+, and Ca+, which are detectable in the UV. It will empirically test non-LTE (local thermodynamic equilibrium) calculations by providing a robust benchmark abundance for them to match: the abundance derived from weak lines of the ions, which are formed in LTE. Results will be compared against approximately 10 previous NLTE calculations for Mg, Si, or Ca, and will be used to assess Galactic chemical evolution model predictions. A laboratory experiment component will anchor theoretical branching fractions for the UV Si II lines to calibrate them to an absolute experimental scale. This work will rely on archival UV spectra originally collected for other purposes, thereby enabling a new, potentially transformative application from these legacy data. I.U. Roederer and C. Sneden have experience performing detailed abundance analyses on late-type stars, like those described in the proposed activities. Both also have experience working with high-resolution STIS UV spectra. J.E. Lawler and E.A. Den Hartog have experience in laboratory atomic spectroscopy and interpreting the literature on this subject to yield precise and accurate atomic data. J.J. Cowan has experience interpreting abundance data in the context of nucleosynthesis and Galactic chemical evolution. The team members have been responsible for > 20 peer-reviewed publications making use of STIS or COS data in the last ~15 years. Stellar Physics and Stellar Types 0 Regular false false true false false SUPPORTED