The Lithium Problem(s)

Lithium measured in low-metallicity halo-stars shows a so called "Spite plateau", which is thus believed to be the evidence of primordial lithium. This is a reasonable conclusion being that lithium is very fragile and the Big Bang nucleosynthesis is the only mechanism able to produce lithium abundance close to the observed plateau. However, recent WMAP measurements of baryon density result in a primordial 7Li abundance that is about two times higher then the "Spite plateau", hence "the lithium problem". Moreover, any pre-galactic source of lithium would contribute to the value observed in low-metallicity halo-stars, and will have to be corrected for, which amplifies the lithium problem.
Besides in the Big Bang, lithium is also produced in cosmic-ray interactions. A cosmological cosmic-ray population that arises from the process of large-scale structure formation will be the source of pre-Galactic lithium production, thus making the lithium problem worse. We have taken a model-independent approach by using the lithium-gamma-ray connection in order to constrain lithium production by structure-formation cosmic-rays. We found that present limits on the extragalactic gamma-ray background allow for significant production of pre-galactic lithium by SFCRs.

Unlike 7Li, the rare isotope 6Li is mainly produced in cosmic-ray interactions. The standard picture is that the observed 6Li is produced by galactic cosmic-rays accelerated in supernova remnants. Using our lithium-gamma-ray connection we tested this assumption and came to a surprising and alarming conclusion: extragalactic gamma-ray background allows for only ~50% of solar lithium-6 abundance to be produced by Galactic Cosmic Rays (in the standard picture). Our calculation uses a carefully propagated cosmic-ray spectrum and accounts for 6Li production from both fusion reactions ($\alpha \alpha \to ^6Li$) as well as from spallation channels (${p,\alpha+CNO \to ^6Li$). We find that although extreme assumptions yield a consistent picture, more realistic ones indicate that solar 6Li cannot be produced by standard GCRs alone without overproducing the hadronic gamma rays. Could this be another lithium problem?

Reference: Fields, B. D., & Prodanović, T. 2005, ApJ 623, 877 (get from astro-ph/)
                   Prodanović, T. & B. D. Fields 2006, ApJL accepted (get from astro-ph/)

Lithium in High-Velocity Clouds

Pre-Galactic Li currently can only be observed in low metallicity Galactic halo stars, but abundance measurements are plagued with systematic uncertainties due to modeling of stellar atmospheres and convection. We propose a new site for measuring pre-Galactic Li: low-metallicity, high-velocity clouds (HVCs) which are likely to be extragalactic gas accreted onto the Milky Way, and which already have been found to have deuterium abundances consistent with primordial. A Li observation in such an HVC would provide the first extragalactic Li measurement, and would be an important step towards resolving the lithium problem. Furthermore, HVC Li could at the same time test for the presence of non-primordial Li due to cosmic rays.

Reference: Prodanović, T. & Fields, B. D. 2004, ApJL 616, 115 (get from astro-ph/)