The barium isotopic fractions in five metal-poor stars

Abstract

Context. Theory and observations of heavy element nucleosynthesis are in conflict with one-another. Theory states that in the most metal-poor stars, the rapid (r-) neutron-capture nucleosynthetic process would be dominant over the slow (s-) process. The most recent determinations of r- and s-process yields do not support this. Aims: We provide measurements of the Ba isotopic fractions for five metal-poor stars derived with a local thermodynamic equilibrium (LTE) analysis with 1D model stellar atmospheres. This increases the comparisons with heavy element nucleosynthesis theory. Methods: We use high resolution (R ≡ λ/Δλ = 90 000-95 000), very high signal-to-noise (S/N > 500) spectra to determine the fraction of odd Ba isotopes (fodd) by measuring subtle asymmetries in the profile of the Ba ii line at 4554 Å. We also use two different macroturbulent broadening techniques, Gaussian and radial-tangential, to model the Fe lines of each star, and propagate each technique to model macroturbulent broadening in the Ba 4554 Å line. We conduct a 1D non-LTE (NLTE) treatment of the Fe lines in the red giant HD 122563 and the subgiant HD 140283 in an attempt to improve the fitting. We determine [Ba/Eu] ratios for the two giants in our study, HD 122563 and HD 88609, which can also be used to determine the relative contribution of the s- and r-processes to heavy-element nucleosynthesis, for comparison with fodd. Results: We find mathematical solutions of fodd for HD 122563, HD 88609 and HD 84937 of -0.12 ± 0.07, -0.02 ± 0.09, and -0.05 ± 0.11 respectively. BD + 26° 3578 yielded a value for fodd = 0.08 ± 0.08. Only BD - 04° 3208 was found to have a physical fodd ratio of 0.18 ± 0.08. This means that all stars examined here show isotopic fractions more compatible with an s-process dominated composition. The [Ba/Eu] ratios in HD 122563 and HD 88609 are found to be -0.20 ± 0.15 and -0.47 ± 0.15 respectively, which indicate instead an r-process signature. We report a better statistical fit to the majority of Fe profiles in each star when employing a radial-tangential broadening technique during our 1D LTE investigation. Conclusions: With the increase of the number of stars for which the Ba isotope fraction fodd has been measured, and the nature of their results, there is now a stronger argument to suggest that other synthesis codes that employ alternative approaches to radiative transfer (e.g. 3D hydrodynamics) have to be considered to tackle the high level of precision required for the determination of isotopic ratios. We have shown that, from a statistical point of view, one must consider using a radial-tangential broadening technique rather than a Gaussian one to model Fe line macroturbulences when working in 1D. No improvement to Fe line fitting is seen when employing a NLTE treatment of the Fe lines.