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Astron. Astrophys. 322, 109-115 (1997)

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Lithium observations in 47 Tucanae *

L. Pasquini 1 and P. Molaro 2

1 European Southern Observatory, Casilla 19001, Santiago 19, Chile
2 Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34131 Trieste, Italy

Received 3 October 1996 / Accepted 21 November 1996

Abstract

We present high resolution observations
(FWHM=15 km s-1) of the Li region in 5 stars belonging to the old ([FORMULA] 14 Gyrs), metal rich ([Fe/H] [FORMULA] -0.7) globular cluster 47 Tuc. At the ESO NTT telescope we obtained EMMI spectra for three V [FORMULA] 17.4 magnitude stars located at the turnoff with [FORMULA] on the Spite plateau. In two of the turnoff stars the lithium line is clearly detected and the mean lithium content [Li] = 2.37 [FORMULA] 0.08 [FORMULA] 0.07 is derived. This value is slightly higher than the Spite plateau at [Li]=2.19 [FORMULA] for field halo stars obtained with the same [FORMULA] scale by Bonifacio and Molaro (1996).

For the third turnoff star only an upper limit could be derived: [Li] [FORMULA] 2.19. This upper limit may suggest the presence of some dispersion on the plateau stars of 47 Tuc as it is observed in field stars of similar metallicity.

When compared with field dwarfs of [Fe/H] [FORMULA] -0.7 the Li content of 47 Tuc turnoff stars is among the highest observed. Considering the extreme age of 47 Tuc 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 47 Tuc stars with metallicities up to [FORMULA]. 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] [FORMULA] 0.7 and [FORMULA] -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 NGC 6397 by Pasquini and Molaro (1996).

Key words: stars: abundances – stars: Population II – globular clusters: individual: 47 Tuc – Galaxy: halo – cosmology: observations

* Based on observations collected at ESO, La Silla

Send offprint requests to: Luca Pasquini

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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