Séminaires : Documents

The temperature and chronology of heavy-element nucleosynthesis in low-mass stars
Sophie Van Eck (ULB)

Asymptotic giant branch (AGB) stars are essential contributors to the heavy-element enrichment of the interstellar medium, because they are responsible for the production of roughly half of the elements heavier than iron and because they eject their envelope through strong stellar winds at the end of their lives. Extrinsic AGB stars are binaries, they are not located on the AGB but bear the unaltered signature of an ancient AGB nucleosynthesis that once took place in their companion star. Detailed abundance determinations in both AGB and extrinsic AGB stars allow to put strong new constrains on the s-process nucleosynthesis taking place in evolved stars. The s-process builds up heavier elements that are subsequently brought to the stellar surface by convection. Two neutron sources, activated at distinct temperatures, have been proposed: 13C and 22Ne. To explain the measured stellar abundances, stellar evolution models invoking the 13C neutron source (which operates at temperatures of about one hundred million kelvin) are favored. Isotopic ratios in primitive meteorites, however, reflecting nucleosynthesis in the previous generations of stars that contributed material to the Solar System, point to higher temperatures (more than three hundred million kelvin), requiring at least a late activation of 22Ne. We determine the s-process temperature directly in evolved low-mass giant stars, using zirconium and niobium abundances, independently of stellar evolution models. The radioactive pair 93Zr-93Nb used to estimate the s-process temperature also provides, together with the pair 99Tc-99Ru, chronometric information on the time elapsed since the start of the s-process.