Title: Heavy Metal from Ancient Superstars
Speaker: Caty Pilachowski
Abstract
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.