From first stars to the Spite plateau: A possible reconciliation of halo stars observations with predictions from big bang nucleosynthesis

Since the pioneering observations of Spite & Spite in 1982, the constant lithium abundance of metal-poor ([Fe/H] < -1.3) halo stars near the turnoff has been attributed to a cosmological origin. Closer analysis, however, revealed that the observed abundance lies at Delta Li-7 similar to 0.4 dex below the predictions of big bang nucleosynthesis (BBN). The measurements of deuterium abundances along the lines of sight toward quasars, and the recent data from the Wilkinson Microwave Anisotropy Probe (WMAP), have independently confirmed this gap. We suggest here that part of the discrepancy (from 0.2 to 0.3 dex) is explained by a first generation of stars that efficiently depleted lithium. Assuming that the models for lithium evolution in halo turnoff stars, as well as the Delta Li-7, estimates are correct, we infer that between one-third and one-half of the baryonic matter of the early halo (i.e., similar to 10(9) M circle dot) was processed through Population III stars. This new paradigm proposes a very economical solution to the lingering difficulty of understanding the properties of the Spite plateau and its lack of star-to-star scatter down to [Fe/H] = -2.5. It is moreover in agreement both with the absence of lithium in the most iron-poor turnoff star currently known (HE 1327-2326) and also with new trends of the plateau suggesting its low-metallicity edge may be reached around [Fe/H] = -2.5. We discuss the role of turbulent mixing associated with enhanced supernovae explosions in the early interstellar medium in this picture. We suggest how it may explain the small scatter and also other recent observational features of the lithium plateau. Finally, we show that other chemical properties of the extremely metal-poor stars (such as carbon enrichment) are also in agreement with significant Population III processing in the halo, provided these models include mass loss and rotationally induced mixing.