Cosmochronometer and r-Process Nucleosynthesis

E-ISSN： 1638-1963|27|issue|133-141

ISSN： 1633-4760

Source： EAS Publications Series, Vol.27, Iss.issue, 2007-09, pp. : 133-141

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Abstract

Nuclear cosmochronometry is a direct method to measure cosmic age.It has been believed that the abundance ratio of Th/Euis a reliable chronometer and Th/Eu ratios of metal-poor stars have been studied intensively to obtain a lower limit of Galactic age.However, after the actinide-boosted metal-poor star CS 31082-001 had been found,applicability of Th/Eu chronometer to metal-poor stars became questionable.Furthermore, theoretical r-process calculations indicate that the Th/Eu production ratios strongly depend on the nucleosynthesis environmentcompared to other stable elements between the second and third peak, including Pb.It is impossible to determine the Th/Eu production ratioby theoretical study at this point, though it is essential for the chronometer.Besides, observed universality of r-process abundances does notguarantee a universal Th/Eu production ratio anymore.Th/Eu chronometer is now facing a serious crisis.On the other hand, the U/Th production ratio obtained by theoretical calculation does not depend on the nucleosynthesis environment significantly.Although there is only one U measurement of metal-poor stars at this point,the U/Th chronometer seems to be still robust.Many uncertainties related to data analyses and nuclear physics modelsstill remain to be resolved.