The End of Nucleosynthesis: Production of Lead and Thorium in the Early Galaxy
Abstract
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 nn <= 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2009
- DOI:
- 10.1088/0004-637X/698/2/1963
- arXiv:
- arXiv:0904.3105
- Bibcode:
- 2009ApJ...698.1963R
- Keywords:
-
- Galaxy: halo;
- nuclear reactions;
- nucleosynthesis;
- abundances;
- stars: abundances;
- stars: Population II;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Galaxy Astrophysics
- E-Print:
- Accepted for publication in the Astrophysical Journal. 22 pages, 10 figures