Lithium and sodium in the globular cluster M4. Detection of a Li-rich dwarf star: preservation or pollution?

Details Lithium and sodium in the globular cluster M4. Detection of a Li-rich dwarf star: preservation or pollution? Lithium and sodium in the globular cluster M4. Detection of a Li-rich dwarf star: preservation or pollution?

2011-07-31

arxiv.org/abs/1108.0138v2

Astronomy and Astrophysics

Astrophysics

Solar and Stellar Astrophysics

8 pages, 1 table, 2 figures. Accepted for publication in A&A. Text modified following referee report, conclusions unchanged

Scientific paper

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.

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