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

Abstract

The oldest stars born before the onset of the main s-process are expected to reveal a pure r-process Ba/Eu abundance ratio. 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. 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. For most investigated stars, NLTE leads to a lower Ba, but a higher Eu abundance. The resulting elemental ratio of the NLTE abundances amounts, 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 Kappeler. We present the NLTE abundance corrections for lines of Ba II and Eu II in the grid of VMP model atmospheres.

12 pages, 8 tables, accepted for publication in A&A

References in corpus (9)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• First Stars VIII -- Enrichment of the neutron-capture elements in the early Galaxy
• Galactic Chemical Evolution and solar s-process abundances: dependence on the 13C-pocket structure
• Near-UV Observations of HD221170: New Insights into the Nature of r-Process-Rich Stars
• Neutron-capture elements in the very metal-poor star HD122563
• Neutron-capture elements in the very metal-poor star HD88609: another st ar with excesses of light neutron-capture elements
• Non-LTE line formation for heavy elements in four very metal-poor stars
• Evolution of the Barium abundance in the early Galaxy from a NLTE analysis of the Ba lines in a homogeneous sample of EMP stars
• Barium even-to-odd isotope abundance ratios in thick disk and thin disk stars

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