2012A&A...539A.157M

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. We aim at investigating the Li and Na content of the GC M4, that is known to have a well defined Na-O anti-correlation. We obtained moderate resolution (R=17000-18700) spectra for 91 main sequence (MS)/sub-giant branch stars of M4 with the Giraffe spectrograph at the FLAMES/VLT ESO facility. Using model atmospheres analysis we measured lithium and sodium abundances. 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, compatible with current estimates of the primordial lithium abundance. 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 NGC6397 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.