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Astron. Astrophys. 341, 539-546 (1999)

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5. Conclusions

We have determined a metallicity of [Fe/H] = [FORMULA] for NGC 6553, together with an overabundance of [FORMULA]-elements, as well as of Na and Al, in the range +0.3 [FORMULA] [X/Fe] [FORMULA] +0.6 (Table 8). Using the overabundances given in Table 8, and assuming [O/Fe] = [S/Fe] = [Ne/Fe] = +0.4 (as we would expect from the overabundances obtained for the other [FORMULA]-elements), we obtain an overall content in heavy elements of Z = 0.014, or [FORMULA], which is almost solar. This is likely to explain why integrated observations tend to give a higher metallicity to NGC 6553, relative to high-resolution observations of FeI lines. As a reference for the metallicity scale of globular clusters, let us point out that [FORMULA] for 47 Tuc (Brown et al. 1990; Brown & Wallerstein 1992). The formal difference is only [FORMULA] dex, and yet the red giant branch of NGC 6553 climbs only [FORMULA] magnitude in V above the horizontal branch, while in 47 Tuc it climbs more than two magnitudes above the horizontal branch (Ortolani et al. 1995). This indicates that the TiO blanketing in the V band is much stronger in NGC 6553 red giants compared to 47 Tuc. At least in part, such a very high sensitivity is likely to be due to both Ti and O being enhanced in NGC 6553, with the abundance of the TiO molecule being proportional to the product of the two abundances.

The abundance ratios in the program stars indicate that the chemical enrichment of the gas from which the bulge cluster NGC 6553 formed was dominated by Type II SNs, with little contamination from long lived stars such as the precursors of Type Ia SNs. Therefore, these findings favour a scenario in which the bulge underwent rapid star formation and chemical enrichment, as also indicated by the colour magnitude diagrams and luminosity functions of bulge globulars and bulge field populations (Ortolani et al. 1995).

Together with the evidence for old stellar populations being dominant in galactic spheroids (ellipticals and bulges), the evidence presented in this paper lends further support to the notion of a much higher star formation rate in the early universe ([FORMULA] compared to the present-day universe (Renzini 1998).

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998