The measurement of the cosmic microwave background has strongly constrained the cosmological parameters of the Universe1. When the measured density of baryons (ordinary matter) is combined with standard Big Bang nucleosynthesis calculations2,3, the amounts of hydrogen, helium and lithium produced shortly after the Big Bang can be predicted with unprecedented precision1,4. The predicted primordial lithium abundance is a factor of two to three higher than the value measured in the atmospheres of old stars5,6. With estimated errors of 10 to 25%, this cosmological lithium discrepancy seriously challenges our understanding of stellar physics, Big Bang nucleosynthesis or both. Certain modifications to nucleosynthesis have been proposed7, but found experimentally not to be viable8. Diffusion theory, however, predicts atmospheric abundances of stars to vary with time9, which offers a possible explanation of the discrepancy. Here we report spectroscopic observations of stars in the metal-poor globular cluster NGC 6397 that reveal trends of atmospheric abundance with evolutionary stage for various elements. These element-specific trends are reproduced by stellar-evolution models with diffusion and turbulent mixing10. We thus conclude that diffusion is predominantly responsible for the low apparent stellar lithium abundance in the atmospheres of old stars by transporting the lithium deep into the star.
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A.J.K. acknowledges a research fellowship by the Leopoldina Foundation, Germany. O.R. thanks the Centre Informatique National de l'Enseignement Supérieur (CINES) and the Réseau Québécois de Calcul de Haute Performance (RQCHP) for providing the computational resources required for this work. F.G. acknowledges financial support from the Instrument Center for Danish Astrophysics (IDA). L.M. acknowledges support through the Presidium RAS Programme 'Origin and evolution of stars and the Galaxy'. The Uppsala group of authors acknowledges support from the Swedish Research Council. We thank A. Alonso and I. Ramirez for providing colour–temperature relations specific to this project.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
Supplementary Figure 1 shows the loci of the observed stars in the observational and physical parameter space. Supplementary Figure 2 displays trends of calcium and titanium as a function of the effective temperatures of the observed star. Supplementary Table 1 compares spectroscopic and photometric effective temperatures of the four groups of stars. (PDF 148 kb)
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Korn, A., Grundahl, F., Richard, O. et al. A probable stellar solution to the cosmological lithium discrepancy. Nature 442, 657–659 (2006). https://doi.org/10.1038/nature05011
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