Accurate Atomic Transition Probabilities for Optical Lines of Fe-group Ions

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

The early Universe was almost completely made up of hydrogen and helium. Stars build up the heavier chemical elements needed for planets and eventually for life through a series of processes known as "nucleosynthesis". The earliest stars were likely quite different than stars today. These old stars, called "metal-poor" stars because of their low abundance of 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. An accurate and precise abundance determination for a given element requires, first and foremost, accurate measurements in our laboratories of the atomic transition properties for that element. The goal of this project is to improve the available atomic data for 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 students and of undergraduates has been, and will continue to be, an important part of this program. The University of Wisconsin (UW) program uses techniques involving lasers, powerful spectrometers, synchrotron radiation, and other modern approaches to measure critically needed atomic data. In recent years, a 3-meter echelle spectrometer was developed at UW to measure very weak lines of Fe-group species in the ultraviolet. Unlike Fourier transform spectrometers, which are often used for such measurements, the 3m echelle spectrometer is free from an important (multiplex) noise source. These weak lines are needed for abundance studies in stellar photospheres, because stronger lines of the abundant Fe-group elements are partially to deeply saturated and are therefore not good abundance indicators. The proposed research will extend the range of the 3m echelle spectrometer into the optical for applications in ground-based astronomy. The first phase of the project will involve developing and implementing a radiometric calibration for the instrument out to the Si detector limit near 10000 Angstroms. The instrument will then be used for the measurement of weak optical lines of astrophysical interest in the spectra of ions of Fe, Co, and Ni. These new transition probabilities will be applied to determine accurate and precise abundances in the Sun and in metal-poor stars.