The lithium content of the globular cluster NGC 6397^*

¹ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Trieste, Via GB Tiepolo 11, 34131 Trieste, Italy
² European Southern Observatory, Karl–Schwarzschild–Str. 2, 85748 Garching bei München, Germany
³ Observatoire de Paris- Meudon, 61 Av. de l'Observatoire, 75014 Paris, France
⁴ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
⁶ Dipartimento di Fisica, Università di Pisa, Piazza Torricelli 2, 56100 Pisa, Italy
⁷ Istituto di Astrofisica Spaziale, CNR, via Fosso del Cavaliere, 00133 Roma, Italy
⁸ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Roma, Italy
⁹ Dipartimento di Astronomia, Università di Padova, vicolo dell'Osservatorio 2, 35122 Padova, Italy
¹⁰ Department of Astronomy, University of Aarhus, Bygning 520, 8000 Aarhus C, Denmark
¹¹ Department of Astronomy and McDonald Observatory, University of Texas at Austin, RLM 15.308, C-1400, Austin, TX, 78712-1083, USA
¹² Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Collurania-Teramo, 64100 Teramo, Italy

Corresponding author: P. Bonifacio, bonifaci@ts.astro.it

Received: 1 March 2002
Accepted: 15 April 2002

Abstract

We make use of high resolution, high signal-to-noise ratio spectra of 12 turn-off stars in the metal-poor globular cluster NGC 6397 to measure its lithium content. We conclude that they all have the same lithium abundance A(Li with a standard deviation of 0.056 dex. We use this result, together with Monte Carlo simulations, to estimate that the maximum allowed intrinsic scatter is of the order of 0.035 dex. This is a new stringent constraint to be fulfilled by stellar models which predict Li depletion. We argue that although a mild depletion of 0.1–0.2 dex, as predicted by recent models, cannot be ruled out, there is no compelling reason for it. This fact, together with the good agreement with the Li abundance observed in field stars, supports the primordial origin of lithium in metal-poor stars. Taking the above value as the primordial lithium abundance implies a cosmic baryonic density which is either or , from the predictions of standard big bang nucleosynthesis. The high baryonic density solution is in agreement with recent results on the primordial abundance of deuterium and and on the estimates derived from the fluctuations of the cosmic microwave background.