Laboratory Spectroscopy for Studies of Metal-Poor Halo Stars

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AST 0205124LawlerThe study of abundances of the heavy elements in metal-poor halo stars is contributing to a more quantitative understanding of stellar nucleosynthesis and Galactic chronometry. The odd-even sawtooth pattern of abundances throughout the lanthanides is examined to separate out thecontributions of the r- and s- processes (synthesis by rapid and slow neutron capture) throughoutthe evolution of the Galaxy. The abundances of the radioactive chronometers Th and U aremeasured along with abundances of other near-lying r-process elements such as Os, Ir and Pt todetermine stellar age and thereby put a lower limit on the Galactic age. Abundance accuracy is key in these studies, and yet the database of transition probabilities on which the abundance determination depends is sorely lacking for many of the heavy elements. Often astronomers must make use of solar gf-values which have many sources of uncertainty, particularly for heavy elements which are relatively weak in comparison to the Fe group in the solar spectrum. Old laboratory measurements and semi-empirical results are also often used, although they are known to have large errors for weaker lines. With modern beam-laser techniques, gf-values can be eliminated as the major source of uncertainty in solar and stellar abundance determinations.Dr. James Lawler and colleagues, at the University of Wisconsin, will conduct laboratory measurements of large sets of accurate, absolute transition probabilities. Transition probabilities will be determined by combining radiative lifetimes with branching fractions. The radiative lifetimes will be measured using time-resolved laser-induced fluorescence on a slow beam of atoms/ions. Branching fractions will be measured using Fourier transform spectroradiometry. Priority will be given to the lanthanide spectrum Nd II, as well as spectra of important chronometers Th II and U II. The gf-values of these spectra have been identified as the most in need of improvement, and as critical in studies of halo stars. As time and resources allow, measurements will continue on the spectra Os I, Pt I and Ir I, which have also been identified as needing improved gf-values, and as important in studies of halo stars.****