Li and Be in turnoff stars of globular clusters

Abstract

Lithium abundance in Turnoff stars of Globular Clusters (GC) provide precious information about primordial nucleosynthesis and Globular Cluster formation. Out of the three GC so far observed in some detail, the metal poor NGC6397 shows a constant Li abundance, at the same level of the Spite' plateau; while the more metal rich NGC6752 and 47 Tuc show a beautiful Li-Na anticorrelation (the first of this kind observed in PoP II stars), suggesting the presence of gas processed by a previous generation of stars. These observations are quite puzzling: while they are qualitatively compatible with the scenario of contamination from intermediate mass AGB stars, no progenitor can quantitatively reproduce the observations. Beryllium has also been detected in NGC6397 Turnoff stars, indicating that the gas which formed the stars was exposed for about 200 Myr to the Galactic Cosmic Rays. The emerging picture seems to require that in the early Galaxy both local (SNae, AGB stars) and global (Galactic Cosmic Rays) enrichment processes were acting simultaneously in the star formation phase of the halo.