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The Messenger 117
September 2004


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VLT Observations of Beryllium in a Globular Cluster:
A clock for the early Galaxy and new insights into Globular Cluster formation

L. Pasquini [1], P. Bonifacio [2], S. Randich [3], D. Galli [3], R. G. Gratton [4]

[1] European Southern Observatory, Garching, Germany;
[2] Instituto Nazionale de Astrofisica, Osservatorio Astronomico di Trieste, Italy;
[3] Instituto Nazionale de Astrofisica, Osservatorio Astronomico di Arcetri, Italy;
[4] Instituto Nazionale de Astrofisica, Osservatorio Astronomico di Padova, Italy

Abstract: The first-ever measurement of the beryllium content in two stars of a globular cluster obtained with UVES at the VLT shows that beryllium can be used as a powerful cosmochronometer to date the oldest stars. In this way we estimate that the globular cluster NGC 6397 formed 0.2–0.3 Gyrs after the onset of star formation in the Milky Way. Assuming that this started shortly after the Big Bang (13.7 Gyrs ago according to WMAP), and the subsequent “dark ages” which are thought to last about 0.2 Gyr, the NGC 6397 stars were therefore born ~13.2–13.3 Gyrs ago, a result which is consistent with the age of the cluster as independently derived from main sequence fitting. This consistency would indicate a remarkable agreement between stellar evolution, cosmic-ray nucleosynthesis and cosmology. The UVES spectra prove as well that the gas which formed the stars must have gone through CNO processing in the protocluster cloud before their formation.

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