Thorium and Uranium Chronometers Applied to CS 31082-001

By Hendrik Schatz, R. Toenjes, B. Pfeiffer, Timothy C. Beers, J. Cowan, V. Hill, K. Kratz. 2001

135 citations recorded by the source corpus.

We use the classical r-process model to explore the implications of the recently reported first observation of U in the extremely metal-poor, r-process element-enriched halo star CS 31082-001 for U and Th cosmochronometry. Using updated nuclear physics input and performing a new, conservative, analysis of the remaining uncertainties in the classical r-process model, we confirm that U (together with Th) abundance observations in metal-poor stars are a promising tool for dating r-process events in the early Galaxy, independent of assumptions on Galactic chemical evolution. We show that nuclear physics uncertainties limit the present accuracy of estimated U/Th ages to about 2 Gyr. Critical nuclear data that are required to lower this uncertainty include β-delayed fission branchings and reliable predictions of the onset of deformation in the vicinity of the N = 184 shell closure around 244Tl, as both directly affect predicted U/Th ratios in r-process models. In this paper we apply, for the first time, the new HFBCS-1 mass model within the framework of the classical r-process model. We find that the predicted U and Th abundances are incompatible with the solar U and Th abundances and trace this back to a different prediction of the onset of deformation around 244Tl. In the case of CS 31082-001, we find it likely that the zero-age U and Th abundances were enhanced by about a factor of 2.5 compared to both (1) a theoretical extrapolation from the observed stable elements using the classical r-process model and (2) the zero-age abundances of Th and U in other r-process-enhanced, metal-poor halo stars. Although presently ad hoc, this "actinide boost" assumption solves the apparent problem of the relative age difference compared with other metal-poor halo stars and, at the same time, the problem of the inconsistency of ages based on U/(stable nucleus), Th/(stable nucleus) and U/Th ratios. There clearly exist differences, among some r-process-enhanced, metal-poor stars, in the level of the elemental abundances of actinides beyond the third r-process peak. Whether CS 31082-001 is a relatively rare case or commonplace awaits the identification of larger numbers of r-process-enhanced, metal-poor stars in which both U and Th can be measured. Using the U/Th ratio, we obtain a best age estimate for the r-process elements in CS 31082-001 of 15.5 ± 3.2 Gyr. Future observations of Pb and Bi and a better determination of the r-process contribution to solar Pb are needed to put the age estimates for this and other stars on a more solid basis. For our most likely scenario, we provide predictions of the expected upper and lower limits on the abundances of the elements Pb and Bi in CS 31082-001.

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1. C. Sneden
2. A. Frebel
3. Timothy C. Beers
4. S. Bisterzo
5. Roberto Gallino
6. Wak Aoki
7. A. Karakas
8. Satoshi Honda

Citations in the corpus

• Nuclear weak-interaction processes in stars (2002, methodology): “The effect of different nuclear physics input on the r-process production of U and Th have been studied by Goriely and Arnould (2001) and by Schatz et al. (2002) As relevant nuclear input, r-process simulations require neutron separation energies (i.e. masses), half-lives and neutron capture cross…”
• THE DISCOVERY AND ANALYSIS OF VERY METAL-POOR STARS IN THE GALAXY (2005, background): “…involved, which depends ultimately on detailed understanding of the operation of the r-process itself (a nucleosynthesis path that is not yet firmly associated with a given astrophysical site), as well as on uncertain nuclear physics constraints (see, e.g., Schatz et al. 2002, Wanajo et al. 2002).”
• Near-Ultraviolet Observations of HD 221170: New Insights into the Nature of r-Process-rich Stars (2006, methodology): “While the abundances of the stable elements (through the 3rd r-process peak) in CS 31082-001 are consistent with the predicted scaled solar system r-process distribution, Th and U are enhanced with respect to the other n-capture elements (Hill et al. 2002; Schatz et al. 2002).”
• The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, background): “Ba to Bi follow the scaled Solar-system r-element abundances (Schatz et al. 2002; Roederer et al. 2009). c© 2002 RAS, MNRAS 000, 1–35 Starting from the pioneering work of Burbidge et al. (1957), it is clear that s- and r-process have to be ascribed to separate astrophysical sites.”
• Neutron-Capture Elements in the Early Galaxy (2008, methodology): “This in turn depends on the nuclear mass formulae and r-process models employed in making those determinations (see discussion in Schatz et al. 2002 and Cowan & Sneden 2004).”
• Spectroscopic Studies of Extremely Metal-Poor Stars with the Subaru High Dispersion Spectrograph. II. The r-Process Elements, Including Thorium (2004, methodology): “…from the need to accurately predict the initial production ratios, which in turn depend on having a detailed understanding of the nucleosynthesis pathways, nuclear mass models, and cross sections for species that have not been measured adequately at present (see Schatz et al. 2002 for details).”
• Nucleosynthesis in the Early Galaxy (2007, background): “The universality of the abundance pattern of the heavy r-process elements seems not to extend to the actinides Th and U, where some star to star scatter has been found in some cases (Hill et al. 2002; Goriely & Arnould 2002; Schatz et al. 2002; Honda et al. 2004).”
• The Extremely Metal-poor, Neutron Capture-rich Star CS 22892-052: A Comprehensive Abundance Analysis (2003, background): “An average of several chronometric ratios suggests an overall chronometric age estimate of 14:2 ’ 3 Gyr for CS 22892-052.”
• Probing the Neutron‐Capture Nucleosynthesis History of Galactic Matter (2002, methodology): “(See also Schatz et al. 2002 for new age determinations of CS 31082-001 employing another nuclear mass model.)”
• Discovery of HE 1523–0901, a Strongly r-Process-enhanced Metal-poor Star with Detected Uranium (2007, methodology): “A; rhee@srl.caltech.edu The similarities of the neighboring nuclei Th (Z=90) and U (Z=92) causes this cosmic clock to be much less prone to systematic uncertainties than the more widely used Th/Eu (Z=63 for Eu) chronometer (Goriely & Clerbaux 1999; Schatz et al. 2002).”