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 3 ^ { rd } 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 \leq log n _ { n } \leq 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 3 ^ { rd } 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 .