The present paper analyzes critically the impact of the remaining uncertainties in the existing r-process models on the determination of the age of the Galaxy derived on grounds of the Th cosmochrometry. Although the direct and accurate observation of the Th abundance in ultra-metal-poor stars provides a possible method to date the age of the star without calling for complex model of the chemical evolution of the Galaxy, the Th cosmochronometry remains affected by all the difficulties associated with our poor understanding of the r-process nucleosynthesis. Even if we disregard the difficulty associated with the fundamental assumption relative to the unicity of the r-process site in the Galaxy, it is impossible at the present stage reliably to determine the age of a ultra-metal-poor star from parametric models of the r-process. The major difficulties lie in the very s- or r-origin of the Pb and Bi isotopes, in addition to the unavoidable problem related to the prediction of nuclear data (principally masses) and the still unknown thermodynamic conditions in which the r-process takes place (in particular concerning the maximum neutron irradiations that can be reached). New accurate observations including Pb and Th lines in ultra-metal poor stars could shed light on the ability of the r-process to produce the Pb-peak elements, and consequently constrain the Th cosmochronometry. In particular, the Th cosmochronometry could be put on safer grounds, especially if Th and U lines could be observed accurately and simultaneously in ultra-metal-poor stars. As regards nuclear uncertainties, only theoretical as well as experimental effort to come can help us achieve the required accuracy to pretend to derive the age of the Galaxy on the basis of the actinides cosmochronometry.
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999