The Lithium Test for Multiple Populations in Globular Clusters: Lithium in NGC 2808

Abstract

In the globular cluster (GC) NGC 2808, a quasi-standard initial lithium abundance is derived for a red giant belonging to the “extreme” population, characterized by a large helium overabundance and by abundances of proton-capture elements typical of nuclear processing in gas at very high temperatures, where the initial lithium has been fully destroyed. The observations of lithium in such extreme cluster stars are important to test different models for the formation of multiple populations in old GCs. In the asymptotic giant branch (AGB) scenario, fresh lithium is synthesized during the initial phases of hot bottom burning which, afterwards, synthesize the other p-capture elements. We model the abundance of lithium in the ejecta of super-AGB models, finding values consistent or larger than observed in the “extreme” giant; these same models describe correctly the magnesium depletion and silicon enrichment of the extreme population of NGC 2808, so the overall agreement provides further support to the AGB scenario. In the models involving massive or supermassive stars, the lithium observed requires a mixture of the lithium-free ejecta of the polluting population with more than 40% of standard-lithium pristine gas. The extended chemical anomalies of NGC 2808 stars are thus all explained within at most 60% of the possible dilution range, the initial helium mass fraction in the ejecta should be Y  ≳ 0.5, to account for the Y e  ∼ 0.38–0.40 of the extreme population, and further observations of p-capture elements are needed to check the model.