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Caffau, E.
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article
Sulfur in the globular clusters <ASTROBJ>47 Tucanae</ASTROBJ> and <ASTROBJ>NGC 6752</ASTROBJ>
Abstract
Context: The light elements Li, O, Na, Al, and Mg are known to show star-to-star variations in the globular clusters <ASTROBJ>47 Tuc</ASTROBJ> and <ASTROBJ>NGC 6752</ASTROBJ>. Such variations are interpreted as coming from processing in a previous generation of stars. <BR />Aims: In this paper we investigate the abundances of the α-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 <ASTROBJ>Terzan 7</ASTROBJ>, which belongs to the <ASTROBJ>Sgr dSph</ASTROBJ>.<BR />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 CO^5BOLD hydrodynamic models and apply corrections due to NLTE effects taken from the literature.<BR />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. <BR />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.Based on observations made with the ESO VLT-Kueyen telescope at the Paranal Observatory, Chile, in the course of the ESO-Large Programme 165.L-0263.