Explorations of the r-Processes: Comparisons between Calculations and Observations of Low-Metallicity Stars

Abundances of heavier elements (barium and beyond) in many neutron-capture-element-rich halo stars accurately replicate the solar system r-process pattern. However, abundances of lighter neutron-capture elements in these stars are not consistent with the solar system pattern. These comparisons suggest contributions from two distinct types of r-process synthesis events, a so-called main r-process for the elements above the second r-process peak and a weak r-process for the lighter neutron-capture elements. We have performed r-process theoretical predictions to further explore the implications of the solar and stellar observations. We find that the isotopic composition of barium and the elemental Ba/Eu abundance ratios in r-process-rich low-metallicity stars can only be matched by computations in which the neutron densities are in the range 23 ≲ log n_n ≲ 28, values typical of the main r-process. For r-process conditions that successfully generate the heavy element pattern extending down to A = 135, the relative abundance of ¹²⁹I produced in this mass region appears to be at least ~90% of the observed solar value. Finally, in the neutron number density ranges required for production of the observed solar/stellar third r-process-peak (A ≈ 200), the predicted abundances of interpeak element hafnium (Z = 72, A ≈ 177-180) follow closely those of third-peak elements and lead. Hf, observable from the ground and close in mass number to the third r-process-peak elements, might also be used as part of a new nuclear chronometer pair, Th/Hf, for stellar age determinations.