Browsing by Subject "Nucleosynthesis"
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Item From heavy atoms to the outer galaxy : characterizing the chemistry of the Milky Way halo(2010-08) Roederer, Ian Ulysses; Sneden, Christopher; Bromm, Volker; Lambert, David; Preston, George; Shetrone, MatthewThis dissertation describes our efforts to use the assembly of matter on nuclear scales as a probe of the assembly of matter on Galactic scales. To investigate the former, we characterize the detailed abundance patterns of the heaviest elements found in ancient, metal-poor stars in the Galaxy. In particular, we place new constraints on and identify several new correlations among the nuclei produced by the rapid nucleosynthetic process, which we use to refine current models of the physical conditions of this process. To investigate the latter, we apply our knowledge of stellar nucleosynthesis to examine correlations between the space motions of stars and their compositions, which retain a record of the composition of the interstellar medium where they formed many billions of years ago. Using new high quality stellar spectra collected from McDonald Observatory and Las Campanas Observatory, we confirm the relative chemical homogeneity of a well-known stellar stream and identify several chemical differences between the two major components of the stellar halo of the Galaxy. Each of these results has significant implications for our understanding of how the Galactic halo formed, grew, and evolved. More profoundly, these results indicate that we have not yet fully characterized the cosmic origins of the heaviest elements in the universe and that we will likely need to examine large samples of metal-poor stars at great distances from the Sun to potentially do so.Item Isotopic fractions of europium, barium, and samarium in metal-poor stars(2007-05) Roederer, Ian Ulysses; Sneden, ChristopherWe present our analysis of the isotopic fractions of europium (Eu), barium (Ba), and samarium (Sm) in three metal-poor stars with contrasting nucleosynthetic enrichment histories. We have obtained new high-resolution (R ~ 120,000) and high-S/N (S/N ~ 160-1000) spectra using the 2dCoudé Spectrograph at the 2.7m Harlan J. Smith Telescope at McDonald Observatory. The isotopic fractions are measured by fitting the observed spectra with synthetic spectra, generated using the most recent laboratory measurements of the hyperfine structure constants and isotope shifts. In the metal-poor giant HD 175305, we measure a Sm isotopic fraction that suggests an r-process origin and a Eu isotopic fraction that is compatible with this interpretation. In the very metal-poor giant HD 196944, we measure a Sm isotopic fraction that suggests an s-process origin. We are unable to measure any isotopic fractions for these species in the very metal-poor and n-capture-poor giant HD 122563 due to the weakness of these lines. We suggest that the Sm isotopic fraction may be a better indicator of s- or r-process nucleosynthesis than the Eu isotopic fraction in some cases. We also propose that the combination of several isotopic fraction measurements in the same metal-poor star can allow direct quantification of the relative s- and r-process nucleosynthesis contributions to n-capture material across a range of nuclear mass numbers, and provide a test of s-process nucleosynthesis predictions at the isotopic level.