From First Stars to the Spite Plateau: A Possible Reconciliation of Halo Stars Observations with Predictions from Big Bang Nucleosynthesis

Abstract

Since the pioneering observations of Spite & Spite in 1982, the constant lithium abundance of metal-poor (½Fe/H < À1:3) halo stars near the turnoff has been attributed to a cosmological origin. Closer analysis, however, revealed that the observed abundance lies at Á 7 Li $ 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 Á 7 Li, estimates are correct, we infer that between one-third and one-half of the baryonic matter of the early halo (i.e., $10 9 M ) 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 ½Fe/H ¼ À2:5. It is moreover in agreement both with the absence of lithium in the most iron-poor turnoff star currently known (HE 1327À2326) and also with new trends of the plateau suggesting its lowmetallicity edge may be reached around ½Fe/H ¼ À2:5. 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.

The metal-poor end of the Spite plateau

“…Even rejecting the interpretation (Venn & Lambert 2008) that this star might be a chemically-peculiar evolved object, the unusual photospherical composition of this star has not yet found a satisfactory explanation. Were the composition of HE 1327-2326 to be indeed primordial, its lack of Li would support the Piau et al (2006) suggestion of a pollution by material cycled through massive Pop. III stars.…”

“…Two main mechanisms could again be invoked. Li could be subject to depletion before the currently observed stars are formed (Piau et al 2006), by means of the reprocessing of the primordial gas in a first generation of massive, hot stars. This phenomenon does not appear to be able to explain the entire WMAP/Spite plateau gap, but, removing up to 0.3 dex of the discrepancy could considerably reduce the problem.…”

“…Adopting the Piau et al (2006) hypothesis, one could then envision a scenario in which partial pollution by this astrated material induces varying degrees of Li "depletion" in EMP stars according to how much this reprocessed gas is available locally at the location and time of each stars' formation. A linear fit to EMP stellar Li abundances would then naturally lead to an expected trend in A(Li) with [Fe/H], whose slope would appear steeper the more the sample is limited to low metallicities.…”

Li isotopes in metal-poor halo dwarfs: a more and more complicated story

“…Piau et al (2006) have proposed an early astration in very masssive stars Pop III stars (destroying Li). They could have destroyed 3/4 of the primordial lithium, 50% of the mass of the Galactic halo having been processed in these massive stars.…”

“…• Early astration -According to Piau et al (2006), the primordial 7 Li, (such as computed by the Big Bang) would have been severely destroyed in the Galaxy before the formation of the old metal-poor stars by astration of the matter in massive Pop III stars, lowering the lithium abundance to the observed pristine abundance. But this theory cannot explain that the same pristine abundance of lithium is observed in the Milky Way and in ωCen (the remnant of an accreted dwarf galaxy) which certainly had different astration histories (see section 3.1).…”