The primordial Li abundance derived from giant stars

Abstract

In this contribution we discuss the use of the surface Li abundance in lower RGB stars as alternative diagnostic of the primordial Li abundance. These stars are located in the portion of the RGB after the completion of the First Dredge-Up and before the extra-mixing episode occurring at the RGB Bump magnitude level. They are sensitive to the total Li content left at the end of the Main Sequence phase and are significantly less sensitive to the efficiency of atomic diffusion when compared with dwarf stars. We analysed lower RGB stars in the Galactic Halo and in the globular clusters NGC 6397, NGC 6752 and M4. The final estimates of initial A(Li) span a narrow range of values (between 2.28 and 2.46 dex), in good agreement with the Spite Plateau and confirming the discrepancy with the values obtained from the standard Big Bang nucleosynthesis calculations.Comment: 7 pages, 3 figures, 1 table, to appear in the proceedings of 'Lithium in the Cosmos', Paris, Feb. 27-29, 2012, Memorie della Societa' Astronomica Italiana Supplement