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Lithium in the Globular Cluster NGC 6397: Evidence for dependence on evolutionary status

Most Globular Clusters are believed to host a single stellar populations. They can thus be considered a good place to study the Spite plateau and probe for possible evolutionary modifications of the Li content. We want to determine the Li content of subgiant (SG) and Main Sequence (MS) stars of the old, metal-poor globular cluster NGC 6397. This work was aimed not only at studying possible Li abundance variations but to investigate the cosmological Li discrepancy. Here, we present FLAMES/GIRAFFE observations of a sample of 84 SG and 79 MS stars in NGC 6397 selected in a narrow range of B-V colour and, therefore, effective temperatures. We determine both Teff and A(Li) using 3D hydrodynamical model atmospheres for all the MS and SG stars of the sample. We find a significant difference in the Li abundance between SG stars and MS stars, the SG stars having an A(Li) higher by almost 0.1 dex on average. We also find a decrease in the A(Li) with decreasing Teff, both in MS and SG stars, albeit with a significantly different slope for the two classes of stars. This suggests that the lithium abundance in these stars is, indeed, altered by some process, which is Teff-dependent. The Li abundance pattern observed in NGC 6397 is different from what is found among field stars, casting some doubt on the use of Globular Cluster stars as representative of Population II with respect to the Li abundance. None of the available theories of Li depletion appears to satisfactorily describe our observations.