VLT observations of turnoff stars in the globular cluster NGC 6397

VLT-UVES high resolution spectra of seven turnoff stars in the metal-poor g lobular cluster NGC 6397 have been obtained. Atmospheric parameters and abu ndances of several elements (Li, Na, Mg, Ca, Sc, Ti, Cr, Fe, Ni, Zn and Ba) were derived. The mean iron abundance is [Fe/H] = -2.02, with no star-to-s tar variation. The mean abundances of the alpha-elements (Ca, Ti) and of th e iron-peak elements (Sc, Cr, Ni) are consistent with abundances derived fo r field stars of similar metallicity. Magnesium is also almost solar, consi stent with the values found by Idiart & Thevenin (2000) when non-LTE effect s (NLTE hereafter) are taken into account. The sodium abundance derived for five stars is essentially solar, but one object (A447) is clearly Na defic ient. These results are compatible with the expected abundance range estima ted from the stochastic evolutionary halo model of Argast et al. (2000), in which at the epoch of [Fe/H] similar to -2 the interstellar medium is supp osed to become well-mixed.