John Cowan, U. of Oklahoma
r-Process Abundances in Halo Stars and the Age of the Galaxy
Physics Division Colloquium - 16 Feb 2001

Abundance observations indicate the presence of rapid-neutron capture (i.e., r-process) elements in old galactic halo and globular cluster stars. These observations demonstrate that the earliest generations of stars in the Galaxy, responsible for neutron-capture synthesis and the progenitors of the halo stars, were rapidly evolving. Abundance comparisons among several halo stars show that the heaviest neutron-capture elements (including Ba and heavier) are consistent with a scaled solar system r-process abundance distribution, while the lighter such elements do not conform to the solar pattern. These comparisons suggest at least two r-process sites. The large star-to-star scatter observed in the neutron-capture/iron ratios at low metallicities- which disappears with increasing [Fe/H]- suggests an early, chemically unmixed and inhomogeneous Galaxy. The stellar abundances indicate a change from the r-process to the slow neutron capture (i.e., s-) process at higher metallicities in the Galaxy. The detection of thorium in halo and globular cluster stars offers a promising, independent age-dating technique that can put lower limits on the age of the Galaxy.

ANL Physics Division Colloquium Schedule