Explorations of ther‐Processes: Comparisons between Calculations and Observations of Low‐Metallicity Stars

Abundances of heavier elements (barium and beyond) in manyneutron-capture-element-rich halo stars accurately replicate the solar systemr-process pattern. However, abundances of lighter neutron-capture elements inthese stars are not consistent with the solar system pattern. These comparisonssuggest contributions from two distinct types of r-process synthesis events --a so called main r-process for the elements above the 2nd r-process peak and aweak r-process for the lighter neutron-capture elements. We have performedr-process theoretical predictions to further explore the implications of thesolar and stellar observations. We find that the isotopic composition of bariumand the elemental Ba/Eu abundance ratios in r-process-rich low metallicitystars can only be matched by computations in which the neutron densities are inthe range 23< log n_n < 28, values typical of the main r-process. For r-processconditions that successfully generate the heavy element pattern extending downto A=135, the relative abundance of I129 produced in this mass region appearsto be at least 90% of the observed solar value. Finally, in the neutron numberdensity ranges required for production of the observed solar/stellar 3rdr-process-peak (A~200), the predicted abundances of inter-peak element hafnium(Z=72, A~180) follow closely those of 3rd-peak elements and lead. Hf,observable from the ground and close in mass number to the 3rd r-process-peakelements, might also be utilized as part of a new nuclear chronometer pair,Th/Hf, for stellar age determinations.Comment: 42 pages, 2 tables, 12 figures; To appear in the Astrophysical Journa