Abundance analysis of turn-off and early subgiant stars in the globular cluster 47 Tuc (NGC 104)

Abstract

We used the UVES spectrograph on Kueyen (VLT UT2) to perform abundance analysis of stars at the base of the giant branch (RGB) and at the main sequence turn-off in the globular cluster 47 Tuc (NGC 104). High dispersion spectra (R ≳ 40000) for 3 dwarfs and 9 subgiants were analyzed. We found an overall [Fe/H] value of -0.67 ± 0.01 ± 0.04, rms = 0.05 dex. The mean value obtained from stars at the base of the RGB is virtually the same as that obtained from the dwarfs. Na and O abundances show a star-to-star scatter and are anti-correlated, not unlike previous findings in more metal-poor clusters. The extension of the anticorrelation is not as extreme as found in other clusters, but it is clearly present in unevolved stars. Al abundances do not show a significant spread; this could be indicative that p-captures in high temperature regions are less likely to occur in metal-rich clusters. The [α/Fe] ratios suggest a slight excess of Ti with respect to field stars of similar metallicities. We found an enhanced odd-even effect for Fe-group elements. In particular, the [Mn/Fe] ratio is almost 0.2 dex deficient with respect to the mean value of the dissipative component of field stars in the solar neighborhood. While the theoretical and observational framework for Mn is still poorly understood, the chemistry of 47 Tue, coupled with its kinematics and its somewhat younger age, might give hints favouring an origin in a formely independent, possibly larger, subsystem of our Galaxy.