Thorium and uranium chronometers applied to CS 31082-001

被引:145
作者
Schatz, H [1 ]
Toenjes, R
Pfeiffer, B
Beers, TC
Cowan, JJ
Hill, V
Kratz, KL
机构
[1] Michigan State Univ, Natl Superconducting Cyclotron Lab, E Lansing, MI 48824 USA
[2] Michigan State Univ, Dept Phys & Astron, E Lansing, MI 48824 USA
[3] Johannes Gutenberg Univ Mainz, Inst Kernchem, D-55128 Mainz, Germany
[4] Univ Oklahoma, Dept Phys & Astron, Norman, OK 73019 USA
[5] European So Observ, D-85748 Garching, Germany
基金
美国国家科学基金会;
关键词
Galaxy : abundances; Galaxy : evolution; nuclear reactions; nucleosynthesis; abundances stars : abundances;
D O I
10.1086/342939
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
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 beta-delayed fission branchings and reliable predictions of the onset of deformation in the vicinity of the N=184 shell closure around Tl-244, 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 Tl-244. 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.
引用
收藏
页码:626 / 638
页数:13
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