The Hamburg/ESO R-process Enhanced Star survey (HERES) - IX. Constraining pure r-process Ba/Eu abundance ratio from observations of r-II stars

摘要

Context. The oldest stars born before the onset of the main s-process are expected to have a pure r-process Ba/Eu abundance ratio. Aims. We revised barium and europium abundances of selected very metal-poor (VMP) and strongly r-process enhanced (r-II) stars to evaluate an empirical r-process Ba/Eu ratio. Methods. Our calculations were based on non-local thermodynamic equilibrium (NLTE) line formation for Ba?ii and Eu?ii in the classical 1D MARCS model atmospheres. Homogeneous stellar abundances were determined from the Ba?ii subordinate and resonance lines by applying a common Ba isotope mixture. We used high-quality VLT/UVES spectra and observational material from the literature. Results. For most investigated stars, NLTE leads to a lower Ba, but a higher Eu abundance. The resulting elemental ratio of the NLTE abundances amounts to, on average, log(Ba/Eu) = 0.78±0.06. This is a new constraint to pure r-process production of Ba and Eu. The obtained Ba/Eu abundance ratio of the r-II stars supports the corresponding solar system r-process ratio as predicted by recent Galactic chemical evolution calculations of Bisterzo, Travaglio, Gallino, Wiescher, and K?ppeler. We present the NLTE abundance corrections for the Ba?ii and Eu?ii lines in the grid of VMP model atmospheres.