Explorations of the r-Processes: Comparisons between Calculations and Observations of Low-Metallicity Stars
Abstract
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<~lognn<~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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2007
- DOI:
- 10.1086/517495
- arXiv:
- arXiv:astro-ph/0703091
- Bibcode:
- 2007ApJ...662...39K
- Keywords:
-
- Nuclear Reactions;
- Nucleosynthesis;
- Abundances;
- Astrophysics
- E-Print:
- 42 pages, 2 tables, 12 figures