Primordial and Pre-Galactic Origins of the Lithium Isotopes

摘要

There are currently two cosmological lithium problems: compared with predictions from Big Bang Nucleosynthesis and models for Galactic cosmic ray production, the observed 7Li abundance in Galactic halo stars is too low while the 6Li content is too high. Here we report on new Keck/HIRES observations of the two extremely metal-poor stars G064-012 and G064-37, which both show distortions of the Li I 670.8 nm line consistent with a significant (3) presence of 6Li. If confirmed, these are the lowest metallicity 6Li detections and would suggest a cosmological or pre-Galactic origin for the isotope. Invoking stellar Li depletion to solve the above-mentioned 7Li discrepancy would further increase the inferred initial 6Li abundance by a factor of 10. Possible 6Li production scenarios are decaying/annihilating supersymmetric particles within the first few minutes of the Big Bang and cosmological cosmic rays from the first stars, although stellar flare production can not be ruled out either. Work still remains, however, before one can unequivocally say that 6Li really has been detected in these and other halo stars, in particular whether convective atmospheric motions and non-LTE line formation can mimic the presence of 6Li in the observed line profiles.