2004A&A...428L...9P

In a previous paper we were able to measure the abundance of uranium and thorium in the very-metal poor halo giant BPS CS31082-001, 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 CS31082-001, 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 log(Pb/H)+12=-0.55±0.15dex, one dex below the upper limits given by other authors for the similar stars CS22892-052 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 12-15Gyr is -0.73±0.17dex. 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.