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Light Elements in the Universe Proceedings IAU Symposium No. 268, 2009 Corinne Charbonnel, Monica Tosi, Francesca Primas c⃝2009 International Astronomical Union & Cristina Chiappini, eds. DOI: 00.0000/X0000000000000000X Main-Sequence and sub-giant stars in the Globular Cluster NGC6397: The complex evolution of the lithium abundance J. I. Gonz´alez Hern´andez1,2†, P. Bonificio1,2,3, E. Caffau1, M. Steffen4, H.-G. Ludwig1,2, N. Behara1, L. Sbordone1, R. Cayrel1, and S. Zaggia5 1GEPI, Observatoire de Paris, CNRS, Universit´e Paris Diderot; Place Jules Janssen 92190 Meudon, France2009 email: Jonay.Gonzalez-Hernandez@obspm.fr 2Cosmological Impact of the First STars (CIFIST) Marie Curie Excellence Team 3INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padua 35122, Italy4Astrophysikalisches D-14482 Potsdam, Germany5Observatory de Meudan; Universite Paris VII; Observatoire du Chili; Faculté des Sciences d’Istanbul; Institut pour la Science et l’Innovation (IS); Centre National de Formation en Astronomie (CNFA) 3 Abstract. Thanks to the high multiplex and efficiency of Giraffe at the VLT we have been for the first time able to observe the Li I doublet in Main Sequue stars of a Globular Cluster. At the same time we observed Li in a sample of Sub-Giant stars of the same B-V colour. Our final sample is composed of 84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same temperature range we find that the Li I doublet in SG[astro-ph.GA] stars are systematically larger than those in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis carried out making use of 1D hydrostatic plane-parallel models and 3D hydrodynamical simulations of the stellar atmospheres. We derived the e�ffective temperatures of stars in our sample from Hα fitting. Theoretical profiles were computed using 3D hydrodynamical simulations and 1D ATLAS models. Therefore, we are able to determined 1D and 3D-based effective temperatures. We infer Li abundances taking into account non-local thermodynamical equilibrium effects when using both 1D and 3D models. We find that SG stars have a mean Li abundance higher by 0.1 dex than MS stars. This result is obtained using both 1D and 3D models. We also detect a positive slope of Li abundance with e�ctive temperature, the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide an unambiguous evidence that the Li abundance changes with evolutionary status. The physical mechanisms responsible for this behaviour are not yet clear, and none of the existing models seems to describe accurately these observations. Based on these conclusions, wearXiv:0912.4105v1 believe that the cosmological lithium problem still remains an open question. Keywords. Stars: abundances – Stars: atmospheres – Stars: fundamental parameters – Stars: Population II – (Galaxy:) globular clusters: individual: NGC 6397 † Present address: Dpto. de Astrof´ısica y Ciencias de la Atmosfera, Facultad de Física, Uni- versidad Complutense de Madrid, E-28040 Madrid, Spain Email: jonay@astrax.fis.ucm.es 1 Introduction The determination of the baryonic density from the fluctuations of the cosmic mi- rowave background (CMB) by the WMAP satellite (Spergel et al. 2007, Cyburt et al. 2008) implies a primordial Li abundance which is log(Li/H) + 12 = 2.72 ± 0.06, at least 0.3–0.5 dex higher than the Li abundance determined in metal-poor stars of the Galactic halo (Spite & Spite 1982). Many different models of Li depletion have been proposed to explain discrepancy: (a) Piau et al.(2006) proposed that the first generation of stars, Population III stars, could have processed some fraction of the halo gas, lowering the lithium abundance; (b) other authors suggest that the primordial Li abundance has been uniformly de- pleted in the atmospheres of metal-poor dwarfs by some physical mechanism (e.g. turbulent diffusion as in Richard et al.(2005), Korn et al.(2006); gravitational waves as in Charbonnel & Talon(2005), etc.); and (c) finally, it has been also suggested that the standard Big Bang nucleosynthesis calculations should be revised, possibly with the introduction of new physics as in Jedamzik et al.(2004), Jedamzik(2006), Jittoh et al.(2008), Hisano et al.(2009). Here we present the determination of Li abundances of subgiant (SG) and main-sequence (MS) stars of the cluster NGC 6397. This work provides the first observations of the Li doublet in MS stars of a Globular Cluster.

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