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

¹ Centre for Astrophysics Research, School of Physics, Astronomy & Mathematics, University of Hertfordshire, College Lane, Hatfield Hertfordshire AL10 9AB UK
e-mail: a.gallagher@herts.ac.uk; s.g.ryan@herts.ac.uk
² Department of Astronomy, PO Box 400325, University of Virginia, Charlottesville, VA 22904-4325, USA
e-mail: aeg4x@mail.astro.virginia.edu
³ National Astronomical Observatory, Mitika, Tokyo 181-8588, Japan
e-mail: aoki.wako@nao.ac.jp

Received: 10 May 2010
Accepted: 20 August 2010

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 χ² analysis.

Methods. We have acquired a very high resolution (R ≡ λ/Δλ = 95000), 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 χ² 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 ¹³⁸Ba are synthesized by the r- and s-process while the other even isotopes (^134,136Ba) 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 vsini ≤ 3.9   kms^-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.