The cosmological lithium problem

Abstract

The discrepancy between the predictions of primordial nucleosynthesis and the observed lithium abundance in Spite plateau stars has been attributed either to a challenge to the standard model of nucleosynthesis or to stellar processes occurring after the stars formed. To understand the origin of this discrepancy, it is crucial to link the cosmic star formation rate with a chemical enrichment model that incorporates the yields of both Population (Pop) III and II stars. It is within this framework that the evolution of lithium can be determined. The primary goal is to demonstrate that there is no discrepancy between the predictions of primordial nucleosynthesis and the observed lithium abundance. By combining a standard chemical evolution model with the hierarchical structure formation scenario, it is possible to determine the lithium abundance as a function of $[\mathrm{Fe/H}]$. The model's results are compared with observational data, including two extremely iron-poor stars: J0023+0307 and SMSS J0313-6708. The Spite plateau is naturally established in the range $-8.0 \lesssim [\mathrm{Fe/H}] \lesssim -2.0$ with $^{7}\mathrm{Li/H}$ $\sim 1.81 \times 10^{-10}$. We find that J0023+0307 could have formed $\sim 4.4 \times 10^{5} - 1.3 \times 10^{6}$ years after the explosion of the first Pop III star in the Universe, whereas for SMSS J0313-6708 this event would have occurred $\sim 2.2 \times 10^{5} - 4.4 \times 10^{5}$ years later. The Spite plateau serves as an observational signature of the formation of Pop III stars. The abundances observed in J0023+0307 and SMSS J0313-6708 are consistent with Pop III progenitor stars in the mass range $10-100 M_{\odot}$. However, if some high-redshift star formation occurs within subhalo-like structures, the contribution of stars in the mass range $140-260 M_{\odot}$ to the formation of the extended Spite plateau cannot be ruled out.