Nucleocosmochronology

Nucleocosmochronology is the use of the abundances of radioactive nuclear species and their radiogenic decay daughters to establish the finite age of the elements and the time scale for their formation. The long-lived radioactive nuclei 232Th, 235U, and 238U, formed in the astrophysical r-process of nucleosynthesis, play an important role in this regard. Observational studies of the heavy element abundance patterns in metal deficient stars have provided critical input: (1) the presence of r-process abundances in the oldest stars provides evidence that we are indeed dating the entire epoch of stellar nucleosynthesis in the Galaxy, and (2) the extraordinary agreement of the abundance pattern characteristic of low metallicity stars with the r-process abundance distribution in solar system matter confirms the robustness of the r-process mechanism for the production of nuclei of mass A≳140. We discuss the implications of these abundance studies for nucleocosmochronology and briefly survey recent age determinations.