VLT
observations of turnoff stars in the globular cluster NGC 6397
F. Thévenin, C. Charbonnel, J.A. de Freitas Pacheco, T.P. Idiart, G. Jasniewicz, P. de Laverny, B. Plez
VLT-UVES high resolution spectra of seven turnoff stars in the metal-poor globular cluster NGC 6397 have been obtained. Atmospheric parameters and abundances 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-star variation. The mean abundances of the alpha-elements (Ca, Ti) and of the iron-peak elements (Sc, Cr, Ni) are consistent with abundances derived for field stars of similar metallicity. Magnesium is also almost solar, consistent with the values found by Idiart & Thévenin (2000) when non-LTE effects (NLTE hereafter) are taken into account. The sodium abundance derived for five stars is essentially solar, but one object (A447) is clearly Na deficient. These results are compatible with the expected abundance range estimated from the stochastic evolutionary halo model of Argast et al. (2000), in which at the epoch of [Fe/H] ~ -2 the interstellar medium is supposed to become well-mixed.