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 Hubble Space Telescope /Space Telescope Imaging Spectrograph, Keck/High Resolution Echelle Spectrograph, and Very Large Telescope/UV–Visual Echelle Spectrograph 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 third 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 third 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.