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Description
To alleviate the discrepancy between the prediction of the primordial lithium abundance in the universe and the abundances observed in Pop II dwarfs and subgiant stars, it has been suggested that the stars observable today have undergone photospheric depletion of lithium. To identify the cause of such depletion, it is important to accurately establish the behaviour of lithium abundance with effective temperature and evolutionary phase. Stars in globular clusters are ideal objects for such abundance analysis, as relative stellar parameters can be precisely determined. We conduct a homogeneous analysis of a very large sample of stars in the metal-poor globular cluster NGC 6397, covering well all evolutionary phases from below the main sequence turn-off to high up the red giant branch. Non-LTE Li abundances or abundance upper limits are obtained for all stars, and for a size-able sub-set of the targets also Na abundances are obtained. The sodium abundances are used to distinguish stars formed out of pristine material from stars formed out of material affected by pollution from a previous generation of more massive stars. The dwarfs, turn-off, and early subgiant stars in our sample form a thin abundance plateau, disrupted in the middle of the subgiant branch by the lithium dilution caused by the first dredge-up. A second steep abundance drop is seen at the luminosity of the red giant branch bump. The turn-off stars are more lithium-poor, by up to 0.1dex, than subgiants that have not yet undergone dredge-up. In addition, hotter dwarfs are slightly more lithium-poor than cooler dwarfs, which may be a signature of the so-called Li dip in the cluster, commonly seen among PopI stars. The feature is however weak. A considerably large spread in Na abundance confirms that NGC6397 has suffered from intra-cluster pollution in its infancy and a limited number of Na-enhanced and Li-deficient stars strongly contribute to form a significant anti-correlation between the abundances of Na and Li. It is nevertheless seen that lithium abundances are unaffected by relatively high degrees of pollution. Lithium abundance trends with effective temperature and stellar luminosity are compared to predictions from stellar structure models including atomic diffusion and ad-hoc turbulence below the convection zone. We confirm previous findings that some turbulence, with strict limits to its efficiency, is necessary to explain the observations.
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