The End of Nucleosynthesis: Production of Lead and Thorium in the Early
  Galaxy
  We examine the Pb and Th abundances in 27 metal-poor stars (-3.1 < [Fe/H] <
-1.4) whose very heavy metal (Z > 56) enrichment was produced only by the rapid
(r-) nucleosynthesis process. New abundances are derived from HST/STIS,
Keck/HIRES, and VLT/UVES spectra and combined with other measurements from the
literature to form a more complete picture of nucleosynthesis of the heaviest
elements produced in the r-process. In all cases, the abundance ratios among
the rare earth elements and the 3rd r-process peak elements considered (La, Eu,
Er, Hf, and Ir) are constant and equivalent to the scaled solar system
r-process abundance distribution. We compare the stellar observations with
r-process calculations within the classical "waiting-point" approximation. In
these computations a superposition of 15 weighted neutron-density components in
the range 23 < log(n_n) < 30 is fit to the r-process abundance peaks to
successfully reproduce both the stable solar system isotopic distribution and
the stable heavy element abundance pattern between Ba and U in low-metallicity
stars. Under these astrophysical conditions, which are typical of the "main"
r-process, we find very good agreement between the stellar Pb r-process
abundances and those predicted by our model. For stars with anomalously high
Th/Eu ratios (the so-called actinide boost), our observations demonstrate that
any nucleosynthetic deviations from the main r-process affect--at most--only
the elements beyond the 3rd r-process peak, namely Pb, Th, and U. Our
theoretical calculations also indicate that possible r-process abundance
"losses" by nuclear fission are negligible for isotopes along the r-process
path between Pb and the long-lived radioactive isotopes of Th and U.
