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 ( f odd ) 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 f odd . Results. We find mathematical solutions of f odd 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 f odd = 0.08 ± 0.08. Only BD−04 • 3208 was found to have a physical f odd 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 f odd 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.

Chemical abundances in a high-velocity RR Lyrae star near the bulge

“…For the lines with hyperfine structure (HFS) the total log (g f ) has been listed. Scandium has been split according to online data from Kurucz's hyperfine line list, barium according to Gallagher et al (2012), europium according to Lawler et al (2001) and Ivans et al (2006).…”

“…The lines to synthesize were selected from the line list in Hansen et al (2011) to find the lines that are most likely detectable in RR Lyrae stars. The synthesis was conducted using a line list containing atomic data from , Hansen et al (2013), Bergemann et al (2012), Gallagher et al (2012), Ivans et al (2006), and Lawler et al (2001).…”

The early days of the Sculptor dwarf spheroidal galaxy

“…They all have [Eu/Ba] between 0.48 and 0.79, classically interpreted as the signature of an r-process. Three stars have measured low or moderate f odd : HD 88609 ( f odd = −0.02 ± 0.09, Gallagher et al 2012), HD 122563 ( f odd = 0.22 ± 0.15, Mashonkina et al 2008), and HD 140283 (Collet et al 2009;Gallagher et al 2012, with f odd = 0.15 ± 0.12 and f odd = 0.02 ± 0.06, respectively). The exact value of f odd of HD 140283 continues to foster discussion in the literature.…”

Abundances of carbon-enhanced metal-poor stars as constraints on their formation

“…The adopted dissociation energies are 3.47 eV (CH), 3.42 eV (NH), 6.24 eV (C 2 ), and 7.7 eV (CN). The line list covering the atomic lines (Sr and Ba) are from Sneden et al (2014), with the Sr hyperfine structure from Bergemann et al (2012) and Hansen et al (2013), and the Ba hyperfine structure (HFS) from Gallagher et al (2012). Since most of these stars are of higher metallicities or enriched by an s-process, we used the total solar system isotopic-abundance ratios for the heavy elements.…”