Discovery of three lead-rich stars

About half of the stable nuclei heavier than iron are believed to be synthe sized during the late stages of evolution of stars with masses in the range 0.8-8 solar masses. These elements are then expelled into the interstellar medium through stellar winds after being 'dredged up' towards the surface of the stars. These processes occur when the star is in the 'asymptotic gia nt branch' (AGB) phase of its life. Nuclei (mainly iron) deep inside the st ar slowly capture neutrons and progressively build up heavier elements (the 's-process'). For AGB stars that formed early in the history of the Galaxy , and that therefore have very low abundances of elements heavier than heli um ('metals'), models(1) predict that the s-process will accumulate synthes ized material with atomic weights in the Pb-Bi region. Such stars will ther efore have large overabundances of lead relative to other heavy elements. H ere we report the discovery of large amounts of lead in three metal-poor st ars (HD187861, HD196944 and HD224959). Our analysis shows that these stars are more enriched in lead than in any other element heavier than iron. The excellent agreement between the observed and predicted abundances reinforce s our current understanding of the detailed operation of the s-process deep in the interiors of AGB stars.