IX. Constraining pure r-process Ba/Eu abundance ratio from observations of r-II stars
1 Universitäts-Sternwarte München, Scheinerstr. 1, 81679 München, Germany
e-mail: firstname.lastname@example.org, email@example.com
2 Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya st. 48, 119017 Moscow, Russia
3 Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
Received: 17 February 2014
Accepted: 1 April 2014
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 Käppeler. We present the NLTE abundance corrections for the Ba ii and Eu ii lines in the grid of VMP model atmospheres.
Key words: stars: abundances / stars: atmospheres / nuclear reactions, nucleosynthesis, abundances
Based on observations collected at the European Southern Observatory, Paranal, Chile (Proposal numbers 170.D-0010, and 280.D-5011).
Tables 7 and 8 are available in electronic form at http://www.aanda.org
© ESO, 2014