The Rise of thes‐Process in the Galaxy

From newly obtained high-resolution, high signal-to-noise ratio spectra the abundances of the elements La and Eu have been determined over the stellar metallicity range -3 < [Fe/H] < +0.3 in 159 giant and dwarf stars. Lanthanum is predominantly made by the s -process in the solar system, while Eu owes most of its solar system abundance to the r -process. The changing ratio of these elements in stars over a wide metallicity range traces the changing contributions of these two processes to the Galactic abundance mix. Large s -process abundances can be the result of mass transfer from very evolved stars, so to identify these cases we also report carbon abundances in our metal-poor stars. Results indicate that the s -process may be active as early as [Fe/H] = -2.6, although we also find that some stars as metal-rich as [Fe/H] = -1 show no strong indication of s -process enrichment. There is a significant spread in the level of s -process enrichment even at solar metallicity.