Image: COURTESY ESO, UK SERC/PPARC
Working with a telescope at the European Southern Observatory in Chile, the researchers captured the absorption spectra of three southern starsHD 187861, HD 196944 (see image) and HD 224959and found that all three had an overabundance of lead. "This is the first detection of a lead-star," lead author Sophie Van Eck says. "These stars are almost exclusively enriched with lead. Moreover, the abundances in all three stars show a remarkable similarity."
The high amounts of lead in the stars allow scientists to test the models they use to explain how elements heavier than hydrogen, helium and lithiumlight elements produced immediately by the big bangcame into existence. Elements heavier than iron (which contains 26 protons and 30 neutrons) are produced in stars, the theory goes, through the addition of neutrons. This so-called neutron capture can occur quickly in supernovae by way of the r-process, or it can occur at a more leisurely pace in regular stars near the end of their lifetimes through the s-process.
Because the s-process occurs near the end of a stars lifetime as it burns off helium in its asymptotic giant branch (AGF) phase, it is difficult to study directly. In the new report, the scientists took advantage of binary systems in which one star had already died and was currently a dim white-dwarf. The atmosphere of the younger companion stars picked up much of the dying star's expelled material, thereby providing a signature of the nucleosynthesisthe buildup of heavier elements from lighter onesthat had taken place within it.
According to the accepted model, if the s-process is efficient in stars starting out with a comparatively low metal content, it will produce significant amounts of heavy nuclei up to and including lead. Any further s-processes will then produce unstable, heavier nuclei that will decay back to lead, resulting in an overabundance of the element. "The excellent agreement between predicted and observed abundances reinforces our current understanding of the detailed operations of the s-process in the deep interiors of the stars," Van Eck notes, "and thus constitutes an important piece of information on how the heaviest stable elements in the universe are formed."