Lithium and sodium in the globular cluster M 4
L. Monaco1, S. Villanova2, P. Bonifacio3, E. Caffau4,3⋆⋆⋆, D. Geisler2, G. Marconi1, Y. Momany1 and H.-G. Ludwig4,3
1 European Southern Observatory, 19001 Casilla, Santiago, Chile
2 Universidad de Concepción, 160-C Casilla, Concepción, Chile
3 GEPI, Observatoire de Paris, CNRS, Univ. Paris Diderot, Place Jules Janssen, 92190 Meudon, France
4 Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
Received: 15 July 2011
Accepted: 27 January 2012
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, compatible with 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.
Key words: nuclear reactions, nucleosynthesis, abundances / stars: abundances / stars: Population II / galaxies: abundances / globular clusters: individual: M4 / cosmology: observations
Based on observations taken at ESO VLT Kueyen telescope (Cerro Paranal, Chile, program: 085.D-0537A).
Table A.1 is available in electronic form at http://www.aanda.org
© ESO, 2012