The barium isotopic mixture for the metal-poor subgiant star HD 140283

Abstract

Context. Current theory regarding heavy element nucleosynthesis in metal-poor environments states that the r-process would be dominant. The star HD 140283 has been the subject of debate after it appeared in some studies to be dominated by the s-process. Aims. We provide an independent measure of the Ba isotope mixture using an extremely high quality spectrum and an extensive χ 2 analysis. Methods. We have acquired a very high resolution (R ≡ λ/Δλ = 95 000), very high signal-to-noise (S /N = 1110 around 4554 Å, as calculated in IRAF) spectrum of HD 140283. We exploit hyperfine splitting of the Ba ii 4554 Å and 4934 Å resonance lines in an effort to constrain the isotope ratio in 1D LTE. Using the code ATLAS in conjunction with KURUCZ06 model atmospheres we analyse 93 Fe lines to determine the star's macroturbulence. With this information we construct a grid of Ba synthetic spectra and, using a χ 2 code, fit these to our observed data to determine the isotopic ratio, f odd , which represents the ratio of odd to even isotopes. The odd isotopes and 138 Ba are synthesized by the r-and s-process while the other even isotopes ( 134,136 Ba) are synthesized purely by the s-process. We also analyse the Eu lines. Results. We set a new upper limit of the rotation of HD 140283 at v sin i ≤ 3.9 km s −1 , a new upper limit on [Eu/H] < −2.80 and abundances [Fe/H] = −2.59 ± 0.09, [Ba/H] = −3.46 ± 0.11. This leads to a new lower limit on [Ba/Eu] > −0.66. We find that, in the framework of a 1D LTE analysis, the isotopic ratios of Ba in HD 140283 indicate f odd = 0.02 ± 0.06, a purely s-process signature. This implies that observations and analysis do not validate currently accepted theory. Conclusions. We speculate that a 1D code, due to simplifying assumptions, is not adequate when dealing with observations with high levels of resolution and signal-to-noise because of the turbulent motions associated with a 3D stellar atmosphere. New approaches to analysing isotopic ratios, in particular 3D hydrodynamics, need to be considered when dealing with the levels of detail required to properly determine them. However published 3D results exacerbate the disagreement between theory and observation.

The barium isotopic fractions in five metal-poor stars

“…Both isotope fractions indicate a fully s-process regime, contradicting the Truran (1981) model. However, measurements of f odd in 1D LTE by Lambert & Allende Prieto (2002) and (for the same spectrum) Collet et al (2009) show HD 140283 to be slightly r-process dominated with f odd = 0.30 ± 0.21 and f odd = 0.33 ± 0.13 respectively, which contradicts Magain (1995) and Gallagher et al (2010). When Collet et al (2009) reanalysed the same spectrum again using 3D hydrodynamic model stellar atmospheres they found f odd = 0.15 ± 0.12, indicating an s-process signature and supporting results from Magain (1995) and Gallagher et al (2010).…”

“…For a fully s-process regime, f odd,s = 0.11 ± 0.01, and in a fully r-process regime, f odd,r = 0.46 ± 0.06. Gallagher et al (2010) show a linear relationship between f odd and r-and s-process contributions for Ba determined from Arlandini et al (1999). Values of 0.0 ≤ f odd < 0.11 or 0.46 < f odd ≤ 1.0 are not physical in the context of the nucleosynthesis model but are achievable from more ad hoc isotopic mixes.…”

“…Gallagher et al (2010) find [Fe/H] = −2.59 ± 0.09 and a low [Ba/Fe] ratio = −0.87 ± 0.14, which seemingly point to an r-process origin as Ba is mainly an s-process element in the solar system. Yet according to their isotopic analysis they find f odd = 0.02 ± 0.06, which agrees with the result published by Magain (1995), who finds f odd = 0.08 ± 0.06.…”

“…Issues then arise when complex astrophysical behaviours, such as convection, start to become visible in the high quality stellar data. Basic assumptions used in conventional spectrum synthesis codes that assume a plane-parallel geometry (1D) and local thermodynamic equilibrium (LTE) cannot actually replicate the observed Ba ii 4554 Å line profile (Gallagher et al 2010).…”

The GAPS programme with HARPS-N at TNG

“…We compiled a long list of spectral lines from the literature, starting with Biazzo et al (2012). In particular, our line list com- (2000), Lawler et al (2001b), andDen Hartog et al (2003), where the HFS by Gallagher et al (2010) and Lawler et al (2001a) was adopted for Ba and La, respectively. Solar isotopic ratios by Anders & Grevesse (1989) CNO abundances were measured as follows.…”