L. Monaco, S. Villanova, P. Bonifacio, E. Caffau, D. Geisler, G. Marconi, Y. Momany, H. -G. Ludwig
Context. The abundance inhomogeneities of light elements observed in Globular
Clusters (GCs), and notably the ubiquitous Na-O anti-correlation, are generally
interpreted as evidence that GCs comprise several generations of stars. There
is an on-going debate as to the nature of the stars which produce the
inhomogeneous elements, and investigating the behavior of several elements is a
way to shed new light on this problem. Aims. We aim at investigating the Li and
Na content of the GC M\,4, that is known to have a well defined Na-O
anti-correlation. Methods. We obtained moderate resolution (R=17\,000-18\,700)
spectra for 91 main sequence (MS)/sub-giant branch stars of M\,4 with the
Giraffe spectrograph at the FLAMES/VLT ESO facility. Using model atmospheres
analysis we measured lithium and sodium abundances. Results. We detect a weak
Li-Na anti-correlation among un-evolved MS stars. One star in the sample,
#\,37934, shows the remarkably high lithium abundance A(Li)=2.87, similar to
current estimates of the primordial lithium abundance. Conclusions. The shallow
slope found for the Li-Na anti-correlation suggests that lithium is produced in
parallel to sodium. This evidence, coupled with its sodium-rich nature,
suggests that the high lithium abundance of star #\,37934 may originate by
pollution from a previous generations of stars. The recent detection of a
Li-rich dwarf of pollution origin in the globular cluster NGC\,6397 may also
point in this direction. Still, no clear cut evidence is available against a
possible preservation of the primordial lithium abundance for star #\,37934.
View original:
http://arxiv.org/abs/1108.0138
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