r-Process Abundance Universality and Actinide Cosmochronology

We review recent observational and theoretical results concerning the presence of actinide nuclei on the surfaces of old halo stars and their use as an age determinant. We present model calculations which show that the observed universality of abundances for 56 < Z < 75 elements in these stars does not imply a unique astrophysical site for the r-process. Neither does it imply a universality of abundances of nuclei outside of this range. In particular, we show that a variety of astrophysical $r$-process models can be constructed which reproduce the same observed universal r-process curve for 56 < Z < 75 nuclei, yet have vastly different abundances for Z >= 75 and possibly Z <= 56 as well. This introduces an uncertainty into the use of the Th/Eu chronometer as a means to estimate the ages of the metal deficient stars. We do find, however, that the U/Th ratio is a robust chronometer. This is because the initial production ratio of U to Th is almost independent of the astrophysical nucleosynthesis environment. The largest remaining uncertainties in the U/Th initial production ratio are due to the input nuclear physics models.