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

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1. Introduction

The metal-rich bulge globular clusters are a keystone for the understanding of the formation of the Galactic bulge, which in turn has consequences on the scenarios of galaxy formation.

Globular clusters in the Galactic bulge form a flattened system, extending from the Galactic center to about 4.5 kpc from the Sun (Barbuy et al. 1998). A study of abundance ratios in these clusters is very important for better understanding the formation of the Galactic bulge. The interest in NGC 6553 resides on its location at [FORMULA] [FORMULA] 5.1 kpc, therefore relatively close the sun, and at the same time it is representative of the Galactic bulge stellar population: (a) Bica et al. (1991) and Ortolani et al. (1995) showed that NGC 6553 and NGC 6528 present very similar Colour-Magnitude Diagrams (CMDs), and NGC 6528 is located at [FORMULA] [FORMULA] 7.83 kpc in the Baade Window, very close to the Galactic center; (b) the stellar population of the Baade Window is very similar to that of NGC 6553 and NGC 6528 as shown by Ortolani et al. (1995) by comparing their CMDs and luminosity functions, indicating that they have comparable metallicity and age.

One fundamental issue concerns the star formation timescale in the bulge, the age of bulge stars, and its signature through elemental abundance ratios. A first answer to these questions was given by Ortolani et al. (1995) where it was revealed that the bulge clusters NGC 6528 and NGC 6553 are nearly coeval to the halo clusters. The final value of the ages still depends on the relation between absolute magnitude of horizontal branch (HB) stars and metallicity.

In this work, we address the question of element abundances, in particular the [FORMULA]/Fe ratio in bulge globular clusters. For Baade's Window field stars McWilliam & Rich (1994, hereafter MR) have determined ratios [Mg/Fe] [FORMULA] [Ti/Fe] [FORMULA] +0.3 and [Ca/Fe] [FORMULA] [Si/Fe] [FORMULA] 0.0. From low-resolution spectra for 400 Baade's Window stars, Sadler et al. (1996) have found [Mg/Fe] [FORMULA] 0.3, whereas Idiart et al. (1996) obtained [Mg/Fe] [FORMULA] +0.45 from an integrated spectrum of Baade's Window.

No determination of such abundances in metal-rich bulge clusters is yet available. In Barbuy et al. (1992) the giant member star III-17 was analysed, where [Fe/H] [FORMULA] was obtained; no abundance ratios had been derived due to limitations in S/N.

In this work we present detailed abundance analyses of 2 stars in NGC 6553 using high resolution échelle spectra obtained at the ESO 3.6m telescope. We also discuss this cluster as a calibrator of the metallicity scale of globular clusters.

In Sect. 2 the observations are described. In Sect. 3 the stellar parameters effective temperature, gravity, metallicity and abundance ratios are derived. In Sect. 4 the results are discussed. In Sect. 5 conclusions are drawn.

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998