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    The End Of Nucleosynthesis: Production Of Lead And Thorium In The Early Galaxy

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    Date
    2009-06
    Author
    Roederer, Ian U.
    Kratz, Karl-Ludwig
    Frebel, Anna
    Christlieb, Norbert
    Pfeiffer, Bernd
    Cowan, John J.
    Sneden, Christopher
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    Abstract
    We examine the Pb and Th abundances in 27 metal-poor stars (-3.1 < [Fe/H] < -1.4) whose very heavy metal (Z > 56) enrichment was produced only by the rapid (r-) nucleosynthesis process. New abundances are derived from Hubble Space Telescope/Space Telescope Imaging Spectrograph, Keck/High Resolution Echelle Spectrograph, and Very Large Telescope/UV-Visual Echelle Spectrograph spectra and combined with other measurements from the literature to form a more complete picture of nucleosynthesis of the heaviest elements produced in the r-process. In all cases, the abundance ratios among the rare earth elements and the third r-process peak elements considered (La, Eu, Er, Hf, and Ir) are constant and equivalent to the scaled solar system r-process abundance distribution. We compare the stellar observations with r-process calculations within the classical "waiting-point" approximation. In these computations a superposition of 15 weighted neutron-density components in the range 23 <= log n(n) <= 30 is fit to the r-process abundance peaks to successfully reproduce both the stable solar system isotopic distribution and the stable heavy element abundance pattern between Ba and U in low-metallicity stars. Under these astrophysical conditions, which are typical of the "main" r-process, we find very good agreement between the stellar Pb r-process abundances and those predicted by our model. For stars with anomalously high Th/Eu ratios (the so-called actinide boost), our observations demonstrate that any nucleosynthetic deviations from the main r-process affect-at most-only the elements beyond the third r-process peak, namely Pb, Th, and U. Our theoretical calculations also indicate that possible r-process abundance "losses" by nuclear fission are negligible for isotopes along the r-process path between Pb and the long-lived radioactive isotopes of Th and U.
    Department
    Astronomy
    Subject
    galaxy: halo
    nuclear reactions, nucleosynthesis, abundances
    stars:
    abundances
    stars: population ii
    metal-poor stars
    asymptotic giant branch
    neutron-capture elements
    laboratory transition-probabilities
    astrophysical r-process
    globular-cluster m15
    near-ultraviolet observations
    improved
    oscillator-strengths
    s-process
    low-mass
    astronomy & astrophysics
    URI
    http://hdl.handle.net/2152/35050
    Citation
    Roederer, Ian U., Karl-Ludwig Kratz, Anna Frebel, Norbert Christlieb, Bernd Pfeiffer, John J. Cowan, and Christopher Sneden. "The End of Nucleosynthesis: Production of Lead and Thorium in the Early Galaxy." The Astrophysical Journal, Vol. 698, No. 2 (Jun., 2009): 1963.
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