Variations in the lithium abundances of turn off stars in the globular cluster 47 Tuc

Aims. Our aim is to determine Li abundances in turn off stars of the globular cluster 47 Tuc and test theories about Li variations among turn off stars. Methods. We make use of high resolution (R similar to 43 000), high signal-to-noise ratio (S/N = 50-70) spectra of 4 turn off (TO) stars obtained with the UVES spectrograph at the 8.2 m VLT Kueyen telescope. Results. The four stars observed span the range 1.6 less than or similar to A( Li) less than or similar to 2.14, providing a mean A( Li)= 1.84 with a standard deviation of 0.25 dex. When coupled with data of other two TO stars of the cluster, available in the literature, the full range in Li abundances observed in this cluster is 1.6 less than or similar to A( Li) less than or similar to 2.3. The variation in A( Li) is at least 0.6 dex (0.7 dex considering also the data available in the literature) and the scatter is six times larger than what is expected from the observational error. We claim that these variations are real. A( Li) seems to be anti-correlated with A(Na) exactly as observed in NGC 6752. No systematic error in our analysis could produce such an anti-correlation. Conclusions. Na production through p captures on Ne-22 at temperatures in excess of 3 x 10(7) K and the contemporary Li destruction could result in this anti-correlation. However, such nuclear processing cannot have taken place in the stars themselves, which do not reach such high temperatures, even at their centre. This points towards the processing in a previous generation of stars. The low N/O ratios in the observed stars and the apparent lack of correlation between N an Li abundances, place a strong constraint on the properties of this previous generation. Our results indicate a different behaviour among the globular clusters so far studied, as far as the abundance patterns are concerned.