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Stellar age dating with thorium, uranium and lead

Published online by Cambridge University Press:  01 October 2008

Anna Frebel  and
Karl-Ludwig Kratz
Anna Frebel
Affiliation:
McDonald Observatory and Department of Astronomy, University of Texas, 1 University Station, C1402, Austin TX, 78712 email: [email protected]
Karl-Ludwig Kratz
Affiliation:
Max-Planck-Institut für Chemie (Otto-Hahn-Institut), D-55128 Mainz, Germany email: [email protected]
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Abstract

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We present HE 1523-0901, a metal-poor star in which the radioactive elements Th and U could be detected. Only three stars have measured U abundances, of which HE 1523-0901 has the most confidently determined value. From comparing the stable Eu, Os, and Ir abundances with measurements of Th and U, stellar ages can be derived. Based on seven such chronometer abundance ratios, the age of HE 1523-0901 was found to be ~13 Gyr. Only an upper limit for Pb could be measured so far. Knowing all three abundances of Th, U, and Pb would provide a self-consistent test for r-process calculations. Pb is the beta- plus alpha-decay end-product of all decay chains in the mass region between Pb and the onset of dominant spontaneous fission above Th and U. Hence, in addition to Th/U also Th, U/Pb should be used to obtain a consistent picture for actinide chronometry. From recent r-process calculations within the classical “waiting-point” model, for a 13 Gyr old star we predict the respective abundance ratios of logϵ(Th/U) = 0.84, logϵ(Th/Pb) = −1.32 and logϵ(U/Pb) = −2.16. We compare these values with the measured abundance ratios in HE 1523-0901 of logϵ(Th/U) = 0.86, logϵ(Th/Pb) > −1.0 and logϵ(U/Pb) > −1.9. With this good level of agreement, HE 1523-0901 is already a vital probe for observational “near-field” cosmology by providing an independent lower limit for the age of the Universe.

Keywords

stars: abundancesstars: Population IIGalaxy: halo
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 4 , Symposium S258: The Ages of Stars , October 2008 , pp. 449 - 456
Copyright
Copyright © International Astronomical Union 2009

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