The ⁶Li/⁷Li isotopic ratio in the metal-poor binary CS22876–032^★,★★

¹ Instituto de Astrofísica de Canarias, Vía Láctea, 38205 La Laguna, Tenerife, Spain
e-mail: jonay@iac.es
² Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
³ GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
⁴ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile

Received: 3 June 2019
Accepted: 10 July 2019

Abstract

Aims. We present high-resolution and high-quality UVES spectroscopic data of the metal-poor double-lined spectroscopic binary CS 22876–032 ([Fe/H] approximately −3.7 dex). Our goal is to derive the ⁶Li/⁷Li isotopic ratio by analysing the Li I λ 670.8 nm doublet.

Methods. We co-added all 28 useful spectra normalised and corrected for radial velocity to the rest frame of the primary star. We fitted the Li profile with a grid of the 3D non-local thermodynamic equilibrium (NLTE) synthetic spectra to take into account the line profile asymmetries induced by stellar convection, and performed Monte Carlo simulations to evaluate the uncertainty of the fit of the Li line profile.

Results. We checked that the veiling factor does not affect the derived isotopic ratio, ⁶ Li/⁷Li, and only modifies the Li abundance, A(Li), by about 0.15 dex. The best fit of the Li profile of the primary star provides A(Li) = 2.17 ± 0.01 dex and ⁶ Li/⁷Li = 8₋₅⁺²% at 68% confidence level. In addition, we improved the Li abundance of the secondary star at A(Li) = 1.55 ± 0.04 dex, which is about 0.6 dex lower than that of the primary star.

Conclusions. The analysis of the Li profile of the primary star is consistent with no detection of ⁶ Li and provides an upper limit to the isotopic ratio of ⁶ Li/⁷Li < 10% at this very low metallicity, about 0.5 dex lower in metallicity than previous attempts for detection of ⁶ Li in extremely metal poor stars. These results do not solve or worsen the cosmological ⁷ Li problem, nor do they support the need for non-standard ⁶Li production in the early Universe.