Laboratory Astrophysics for Studies of the Chemical Evolution of the Galaxy

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The early Universe was almost completely made up of hydrogen and helium. Heavier chemical elements are produced in stars through a series of processes known as "nucleosynthesis". The earliest stars were likely quite different from stars today. Old stars, called "metal-poor" stars because they are deficient in heavier elements, were formed from the remnants of the earliest stars. Elemental abundances in metal-poor stars serve as a "fossil record" of our Galaxy and can help us understand the beginnings of nucleosynthesis. Testing theories of the early Universe requires very accurate measurements of elemental abundances in metal-poor stars. Accurate and precise abundance determination for a given element requires, first and foremost, accurate laboratory measurements of the spectral properties of that element. The goal of this research program is to conduct accurate, high-quality measurements of atomic data for the elements and to use these data to determine highly accurate abundances in metal-poor stars. This work could also lead to improvements that benefit other fields of applied science such as lighting and analytical chemistry. Training and mentoring of graduate and undergraduate students has been, and will continue to be, an important part of this program. The Wisconsin Atomic Transition Probability (WATP) program develops and applies advanced experimental techniques to measure accurate, absolute atomic transition probabilities. The transition probabilities are determined by combining branching fractions with radiative lifetimes measured using time-resolved laser-induced fluorescence. In recent years, a 3-meter echelle spectrometer was developed at the University of Wisconsin to measure very weak lines of Fe-group elements at ultraviolet wavelengths but is now optimized to work at optical wavelengths for applications to problems of ground-based astronomy. These weak lines are needed for abundance determination in stellar photospheres, because stronger lines of the abundant Fe-group elements are partially to deeply saturated and are therefore less ideal as abundance indicators. This project will complete work on Fe-group elements, with measurement of transition probabilities of weak lines of Sc II and Fe II. Subsequently, the WATP program will begin work on measuring transition probabilities of the alpha-elements, starting with Ca I. The newly measured transition probabilities will be used to determine accurate and precise abundances in the Sun and in metal-poor stars, and the relative abundances will be compared to models of nucleosynthesis.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.