09 May 2002 CfA Colloquium

Title: Heavy Metal from Ancient Superstars

Speaker: Caty Pilachowski


Old stars in the Milky Way Galaxy record the history of nucleosynthesis in our part of the Universe stretching back almost to the beginning of time. From studies of the compositions of low mass stars that formed more than 10 billion years ago, we can trace the origin of the elements and identify sites of nucleosynthesis. The abundances of heavy metals in these extremely metal-deficient stars follow a different pattern than what is seen in our prototypical modern star, the sun. The lanthanide elements were produced purely by r-process nucleosynthesis, rather than as a mixture of both r- and s-process nucleosynthesis. Significant production of r-process element in the Milky Way appears to have begun as the metallicity of the Galaxy reached [Fe/H]=-2.9. This onset is consistent with the suggestion that Type II supernovae from 8-10 solar mass stars are responsible for the r-process. At lower metallicities and earlier times, the origin of heavy metals is uncertain, but the most metal-deficient stars, at [Fe/H]<-3, contain a mixture of heavy elements unlike anything else we find in stars. The “lighter” heavy elements such as strontium, yttrium, and zirconium, are over-represented with respect to heavier species. Some additional form of the r-process is needed to account for the distribution of heavy metals in the most metal-poor stars. As metallicity continued to increase, contributions from the s-process can first be seen in some stars with metallicities as low as [Fe/H]=-2.75, and are present in most stars with metallicities [Fe/H]>-2.3. The appearance of s-process contributions as metallicity increases reflects the longer stellar evolutionary timescale of the (low-mass) s-process nucleosynthesis sites.