Variations in the lithium abundances of turn off stars in the globular cluster 47 Tucanae^*

¹ CIFIST Marie Curie Excellence Team
² GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France e-mail: Piercarlo.Bonifacio@obspm.fr
³ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁵ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁶ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
⁷ Department of Astronomy, Pontificia Universidad Catolica, Av. Vicuña Mackenna 4860, Santiago 22, Chile

Received: 2 February 2007
Accepted: 17 April 2007

Abstract

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 (), high signal-to-noise ratio (–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 , 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 . 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 ²²Ne at temperatures in excess of  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.