Sulfur in the globular clusters 47 Tucanae and NGC 6752L. Sbordone1, 2, M. Limongi3, A. Chieffi4, E. Caffau2, H.-G. Ludwig1, 2, and P. Bonifacio1, 2, 5
1 CIFIST Marie Curie Excellence Team, France
2 GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
3 INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
4 INAF - IASF, via del Fosso del Cavaliere 100, 00133 Rome, Italy
5 INAF - Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
Received 12 December 2008 / Accepted 8 April 2009
Context. The light elements Li, O, Na, Al, and Mg are known to show star-to-star variations in the globular clusters 47 Tuc and NGC 6752 . Such variations are interpreted as coming from processing in a previous generation of stars.
Aims. In this paper we investigate the abundances of the -element sulfur, for which no previous measurements exist. In fact this element has not been investigated in any Galactic globular cluster so far. The only globular cluster for which such measurements are available is Terzan 7 , which belongs to the Sgr dSph .
Methods. We use high-resolution spectra of the S I Mult. 1, acquired with the UVES spectrograph at the 8.2 m VLT-Kueyen telescope, for turn-off and giant stars in the two globular clusters. The spectra were analysed making use of ATLAS static plane parallel model atmospheres and SYNTHE spectrum synthesis. We also compute 3D corrections from CO5BOLD hydrodynamic models and apply corrections due to NLTE effects taken from the literature.
Results. In the cluster NGC 6752 sulfur has been measured only in four subgiant stars. We find no significant star-to-star scatter and a mean [S/Fe] = +0.49 0.15, consistent with what is observed in field stars of the same metallicity. In the cluster 47 Tuc we measured S in 4 turn-off and 5 subgiant stars with a mean [S/Fe] = +0.18 0.14. While this result is compatible with no star-to-star scatter we notice a statistically significant correlation of the sulfur abundance with the sodium abundance and a tentative correlation with the silicon abundance.
Conclusions. The sulfur-sodium correlation is not easily explained in terms of nucleosynthesis. An origin due to atomic diffusion can be easily dismissed. The correlation cannot be easily dismissed either, in view of its statistical significance, until better data for more stars is available.
Key words: stars: abundances -- Galaxy: globular clusters: individual: NGC 104 -- Galaxy: globular clusters: individual: NGC 6752
© ESO 2009