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We present a homogeneous photometric and spectroscopic analysis of 18 starsalong the evolutionary sequence of the metal-poor globular cluster NGC 6397([Fe/H] = -2), from the main-sequence turnoff point to red giants below thebump. The spectroscopic stellar parameters, in particular stellar-parameterdifferences between groups of stars, are in good agreement with broad-band andStroemgren photometry calibrated on the infrared-flux method. The spectroscopicabundance analysis reveals, for the first time, systematic trends of ironabundance with evolutionary stage. Iron is found to be 31% less abundant in theturnoff-point stars than in the red giants. An abundance difference in lithiumis seen between the turnoff-point and warm subgiant stars. The impact ofpotential systematic errors on these abundance trends (stellar parameters, thehydrostatic and LTE approximations) is quantitatively evaluated and found notto alter our conclusions significantly. Trends for various elements (Li, Mg,Ca, Ti and Fe) are compared with stellar-structure models including the effectsof atomic diffusion and radiative acceleration. Such models are found todescribe the observed element-specific trends well, if extra (turbulent) mixingjust below the convection zone is introduced. It is concluded that atomicdiffusion and turbulent mixing are largely responsible for the sub-primordialstellar lithium abundances of warm halo stars. Other consequences of atomicdiffusion in old metal-poor stars are also discussed.

Atomic Diffusion and Mixing in Old Stars I. VLT/FLAMES-UVES Observations of Stars in NGC 6397