First stars

XV. Third-peak r-process element and actinide abundances in the uranium-rich star CS31082-001^⋆

¹ Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil
e-mail: barbuy@astro.iag.usp.br
² GEPI, Observatoire de Paris, CNRS, UMR 8111, 92195 Meudon Cedex, France
³ Université de Sophia-Antipolis, Observatoire de la Côte d’Azur, CNRS UMR 6202, BP 4229, 06304 Nice Cedex 4, France
⁴ European Southern Observatory, Karl Schwarzschild Strasse 2, 85748 Garching bei München, Germany
⁵ LUPM, CNRS, UMR 5299, Université de Montpellier II, 34095 Montpellier Cedex 05, France
⁶ Technische Universität München, Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
⁷ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
⁸ The Niels Bohr Institute, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁹ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain
¹⁰ Michigan State University, Department of Physics & Astronomy, and JINA: Joinst Institute for Nuclear Physics, East Lansing, MI 48824, USA
¹¹ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy

Received: 9 June 2011
Accepted: 26 July 2011

Abstract

Context. A small fraction of extremely metal-poor (EMP) stars exhibit moderate to extreme excesses of heavy neutron-capture elements produced in the r-process. The production site(s) of these elements in the early Galaxy remain(s) unclear, as is the reason for their occasional enhancement in the otherwise regular pattern of abundances of elements up to the iron peak. The detailed abundance pattern of the heaviest elements in EMP stars provides insight into their origin and role in the chemical enrichment of the early Galaxy and in radioactive nucleochronology.

Aims. The EMP giant star CS 31082-001 ([Fe/H]  ~  −2.9) exhibits an extreme enhancement of neutron-capture elements ([r/Fe]  ~  +1.7) with U and Th enhanced by a further  ~+0.7 dex, and a minimum of blending by molecular lines such as CH or CN. A rich inventory of r-process element abundances was established previously from optical spectra. Here we aim to supplement these data with abundances from near-UV spectroscopy of the third-peak neutron-capture elements, which are crucial for understanding the synthesis of the heaviest elements.

Methods. Near-UV spectra from HST/STIS were analysed with LTE model atmospheres and spectrum synthesis calculations to derive new abundances of Os, Ir, Pt, Au, Bi and Pb in CS 31082-001.

Results. Together with earlier data, the resulting abundance pattern for the r-process elements provides improved constraints on the nature of the r-process. The observed U and Th abundances and the initial production ratio place CS 31082-001 as one of the oldest stars in the Galaxy, consistent with its extreme metal deficiency. Comparison with the heaviest stable reference elements and with the daughter nuclides Pb and Bi provides a consistency check on this age determination. Finally, the existence of such r-element rich stars indicate that the early chemical evolution of the Galaxy was localised and inhomogeneous.