The cosmological lithium problem and light element cosmic evolution with intermediate mass pop III stars

Abstract

We explore the effect of an early population of intermediate-mass (IM) stars in the 2-8 M⊙ range on the cosmic chemical evolution of the light elements. Recent analyses of the 4He abundance in metal-poor extragalactic HII regions suggest an elevated abundance Yp of about 0.256 by mass, higher than the predicted result from the Big bang nucleosynthesis, assuming the baryon density determined by the WMAP, Yp = 0.249. This offset may suggest a prompt initial enrichment of helium in early metal-poor structures by IM stars. We also discuss the effect on the early astration of D and 7Li. Indeed, Standard Big Bang Nucleosynthesis at the baryon density determined by WMAP predicts an excess of 7Li compared to observations (Spite plateau) by a factor of 4-5. In contrast, BBN predictions for D/H are somewhat below (2.6 10−5) of the weighted mean of observationally determined values from quasar absorption systems (3. 10−5). Alteration by the nuclear processes during or subsequent to BBN lead to a significant increase in the deuterium abundance consistent with the highest values of D/H (≃ 4 × 10−5) seen in absorption systems (and a lowering of lithium). We argue that those systems are more representative of the primordial abundance and as a consequence, models of cosmic chemical evolution including early IM SFR are able to destroy in situ Deuterium due to the fragility of this isotope. We conclude that these early intermediate-mass stars possibly associated with Population III stars can explain the observational constraints related to the light cosmological elements.