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Back to the Lithium Plateau with the [Fe/H] < -6 star J0023+0307

Author/s: D. S. Aguado, J. I. González Hernández, C. Allende Prieto, R., Rebolo

Reference: 2019 ApJ 874 L21 | Link

Li abundance, A(Li), versus metallicity, [Fe/H], of the extremely iron-poor dwarf star J0023+0307 compared with other dwarf stars (logg ≥ 3.7) with Li abundance values from Bonifacio et al. (2018) and references therein. Blue filled circles connected with a solid line indicates the spectroscopic binary systems in González Hernández et al. (2008) and Aoki et al. (2012). The Lithium “plateau” (also called Spite Plateau) reference is shown as a solid line at a level of A(Li) = 2.20 dex. Blue dashed line represents the primordial lithium value (A(Li)∼2.7) from WMAP (Spergel et al. 2003).
Li abundance, A(Li), versus metallicity, [Fe/H], of the extremely iron-poor dwarf star J0023+0307 compared with other dwarf stars (logg ≥ 3.7) with Li abundance values from Bonifacio et al. (2018) and references therein. Blue filled circles connected with a solid line indicates the spectroscopic binary systems in González Hernández et al. (2008) and Aoki et al. (2012). The Lithium “plateau” (also called Spite Plateau) reference is shown as a solid line at a level of A(Li) = 2.20 dex. Blue dashed line represents the primordial lithium value (A(Li)∼2.7) from WMAP (Spergel et al. 2003).
We present an analysis of the UVES high-resolution spectroscopic observations at the 8.2m VLT of J0023+0307, an extremely iron-poor dwarf star. We are unable to detect iron lines in the spectrum but derive [Fe/H]< −6.1 from the Ca II resonance lines assuming [Ca/Fe]=0.40. The chemical abundance pattern of J0023+0307, with very low [Fe/Mg] and [Ca/Mg] abundance ratios, but relatively high absolute Mg and Si abundances, suggests J0023+0307 is a second generation star formed from a molecular cloud polluted by only one supernova in which the fall-back mechanism played a role. We measure a carbon abundance of A(C) = 6.2 that places J0023+0307 on the “low” band in the A(C)−[Fe/H] diagram, suggesting no contamination from a binary companion. This star is also unique having a lithium abundance (A(Li)=2.02±0.08) close to the level of the Lithium Plateau, in contrast with lower Li determinations or upper limits in all other extremely iron-poor stars. The upper envelope of the lithium abundances in unevolved stars spanning more than three orders of magnitude in metallicity (−6 <[Fe/H]< −2.5) defines a nearly constant value. We argue that it is unlikely that such uniformity is the result of depletion processes in stars from a significantly higher initial Li abundance, but suggests instead a lower primordial production, pointing to new physics such as decaying massive particles, varying fundamental constants, or nuclear resonances, that could have affected the primordial 7Li production.

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