Letter to the Editor

Lead abundance in the uranium star CS 31082-001^*

¹ GRAAL, CNRS UMR 5024, Université de Montpellier 2, 34095 Montpellier Cedex 5, France
² Observatoire de Paris, GEPI, CNRS UMR 8111, 5 place Jules Janssen, 92195 Meudon Cedex, France
³ Observatoire de Paris, GEPI, CNRS UMR 8111, 61 av. de l'Observatoire, 75014 Paris, France e-mail: roger.cayrel@obspm.fr
⁴ IAG Universidade de São Paolo, Dep. de Astronomia CP 3386, Rua do Matão 1226, São Paolo 05508-900, Brazil
⁵ Department of Physics & Astronomy and JINA, Michigan State University, East Lansing, MI 48824, USA
⁶ Istituto Nazionale per l'Astrofisica – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
⁷ ESO, Karl-Schwarzschild-Str. 2, 85749 Garching bei München, Germany
⁸ Lund Observatory, Box 43, 22100 Lund, Sweden
⁹ Niels Bohr Institute, Juliane Maries Vej 30, 2100 Copenhagen, Denmark

Received: 21 July 2004
Accepted: 21 October 2004

Abstract

In a previous paper we were able to measure the abundance of uranium and thorium in the very-metal poor halo giant BPS CS 31082-001, but only obtained an upper limit for the abundance of lead (Pb). We have got from ESO 17 h of additional exposure on this star in order to secure a detection of the minimum amount of lead expected to be present in CS 31082-001, the amount arising from the decay of the original content of Th and U in the star. We report here this successful detection. We find an LTE abundance dex, one dex below the upper limits given by other authors for the similar stars CS 22892-052 and BD +17° 3248, also enhanced in r-process elements. From the observed present abundances of Th and U in the star, the expected amount of Pb produced by the decay of ²³²Th, and ²³⁸U alone, over 12–15 Gyr is dex. The decay of ²³⁵U is more difficult to estimate, but is probably slightly below the contribution of ²³⁸U, making the contribution of the 3 actinides only slightly below, or even equal to, the measured abundance. The contribution from the decay of ²³⁴U has was not included, for lack of published data. In this sense our determination is a lower limit to the contribution of actinides to lead production. We comment this result, and we note that if a NLTE analysis, not yet possible, doubles our observed abundance, the decay of the 3 actinides will still represent 50 per cent of the total lead, a proportion higher than the values considered so far in the literature.