Lithium observations in 47 Tuc

Abstract

We present high resolution observations (FWHM=15 km/sec) of the Li region in 5 stars belonging to the old (14 Gyrs), metal rich ([Fe/H]=-0.7) globular cluster 47Tuc. At the ESO NTT telescope we obtained EMMI spectra for three V about 17.4 magnitude stars located at the turnoff with Teff on the Spite plateau. In two of the turnoff stars the lithium line is clearly detected and the mean lithium content [Li] = 2.37$+/- 0.08 +/- 0.07 is derived. This value is slightly higher than the Spite plateau at [Li]=2.19 +/- 0.016 for field halo stars obtained with the same Teff scale by Bonifacio and Molaro (1996). For the third turnoff star only an upper limit could be derived: [Li] < 2.19. This upper limit may suggest the presence of some dispersion on the plateau stars of 47Tuc as it is observed in field stars of similar metallicity. When compared with field dwarfs of [Fe/H] about -0.7 the Li content of 47Tuc turnoff stars is among the highest observed. Considering the extreme age of 47Tuc this suggests that no significant depletion affected these stars and it implies a mild Li Galactic enrichment between the epoch of Pop II formation and the formation of 47Tuc stars with metallicities up to about -0.7. Spallation reactions, as deduced from Be observations, can account for such an increase. The remaining two stars are evolved, and only upper limits could be placed at [Li]<0.7 and <-0.7. These very low values imply some extra depletion mechanism in addition to dilution as it is observed in evolved field stars by Pilachowki et al (1993) and in NGC6397 by Pasquini and Molaro (1996).