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The U/Th production ratio and the age of the Milky Way from meteorites and Galactic halo stars


Some heavy elements (with atomic number A > 69) are produced by the ‘rapid’ (r)-process of nucleosynthesis, where lighter elements are bombarded with a massive flux of neutrons1,2,3,4,5,6,7,8. Although this is characteristic of supernovae and neutron star mergers, uncertainties in where the r-process occurs persist because stellar models are too crude to allow precise quantification of this phenomenon. As a result, there are many uncertainties and assumptions in the models used to calculate the production ratios of actinides (like uranium-238 and thorium-232). Current estimates of the U/Th production ratio range from 0.4 to 0.7. Here I show that the U/Th abundance ratio in meteorites9 can be used, in conjunction with observations of low-metallicity stars in the halo of the Milky Way10,11,12, to determine the U/Th production ratio very precisely . This value can be used in future studies to constrain the possible nuclear mass formulae used in r-process calculations5,6, to help determine the source of Galactic cosmic rays, and to date circumstellar grains5. I also estimate the age of the Milky Way ( in a way that is independent of the uncertainties associated with fluctuations in the microwave background13 or models of stellar evolution14,15.

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Figure 1: Distribution of G-dwarf metallicity normalized to solar composition.
Figure 2: Determinations of the U/Th production ratio and the age of the Milky Way.
Figure 3: Estimations of the 238 U/ 232 Th production ratio in r-process nucleosynthesis.

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I thank J. W. Truran, T. Rauscher, A. M. Davis, J. J. Cowan, R. Cayrel, V. Hill, R. Yokochi, B. Marty and L. Reisberg for discussions, and B. Hansen for communication of the 68% confidence interval for the age of the globular cluster Messier 4.

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Correspondence to Nicolas Dauphas.

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This file contains Supplementary Equations S1-S12, Supplementary Figure S1 and Supplementary Table S1 (PDF 184 kb)

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Dauphas, N. The U/Th production ratio and the age of the Milky Way from meteorites and Galactic halo stars. Nature 435, 1203–1205 (2005).

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