A&A 423, 507–516 (2004) Astronomy DOI: 10.1051/0004-6361:20041014 & c ESO 2004 Astrophysics The metal content of the bulge globular cluster NGC 6528, M. Zoccali1,2,B.Barbuy3, V. Hill4, S. Ortolani5,A.Renzini6,E.Bica7,Y.Momany5, L. Pasquini6, D. Minniti1, and R. M. Rich8 1 Universidad Catolica de Chile, Department of Astronomy & Astrophysics, Casilla 306, Santiago 22, Chile e-mail: [mzoccali;dante]@astro.puc.cl 2 Princeton University Observatory, Peyton Hall, Princeton, NJ 08544, USA 3 Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil e-mail: [email protected] 4 Observatoire de Paris-Meudon, 92195 Meudon Cedex, France e-mail: [email protected] 5 Università di Padova, Dipartimento di Astronomia, Vicolo dell’Osservatorio 2, 35122 Padova, Italy e-mail: [ortolani;momany]@pd.astro.it 6 European Southern Observatory, Karl Schwarzschild Strasse 2, 85748 Garching bei München, Germany e-mail: [arenzini;lpasquin]@eso.org 7 Universidade Federal do Rio Grande do Sul, Departamento de Astronomia, CP 15051, Porto Alegre 91501-970, Brazil e-mail: [email protected] 8 UCLA, Department of Physics & Astronomy, 8979 Math-Sciences Building, Los Angeles, CA 90095-1562, USA e-mail: [email protected] Received 2 April 2004 / Accepted 10 May 2004 Abstract. High resolution spectra of five stars in the bulge globular cluster NGC 6528 were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph. Out of the five stars, two of them showed evidence of binarity. The target stars belong to the horizontal and red giant branch stages, at 4000 < Teff < 4800 K. Multiband V, I, J, H, Ks pho- tometry was used to derive initial effective temperatures and gravities. The main purpose of this study is the determination of metallicity and elemental ratios for this template bulge cluster, as a basis for the fundamental calibration of metal-rich popula- tions. The present analysis provides a metallicity [Fe/H] = −0.1 ± 0.2andtheα-elements O, Mg and Si, show [α/Fe] ≈ +0.1, whereas Ca and Ti are around the solar value or below, resulting in an overall metallicity Z ≈ Z. Key words. Galaxy: bulge – globular clusters: individual: NGC 6528 – stars: abundances – stars: atmospheres 1. Introduction as seen in the Baade’s Window field. This is a clear indication that the two populations have comparable age and metallicity. The globular cluster NGC 6528 is located in Baade’s Window, at (J2000) α = 18h04m51.5s, δ = −30o0304 (l = 1.1459◦, For this reason, the two bulge globular clusters (GCs) ◦ b = −4.1717 ), at a distance d = 7.8 kpc from the Sun and NGC 6528 and NGC 6553 have often been used as templates for the old, metal-rich population of the Milky Way central RGC = 0.6 kpc from the Galactic center (Barbuy et al. 1998). spheroid (e.g. Zoccali et al. 2003). Ortolani et al. (1992) first presented BVRI CCD Colour−Magnitude Diagrams (CMD) of NGC 6528. Ortolani Despite its key importance for the interpretation of the et al. (1995) showed that NGC 6528, together with its “twin” formation of our own Galaxy and, by extension, of external NGC 6553, has a Colour Magnitude Diagram (CMD) and a spheroids, the absolute value of the Fe and α-element abun- luminosity function very similar to that of the Galactic bulge, dance, for both NGC 6528 and NGC 6553 still are a matter of debate. The main reasons for this are: i) the intrinsic diffi- culty to observe these faint stars; ii) the ambiguous location of Observations collected both at the European Southern Observatory, Paranal and La Silla, Chile (ESO programme 65.L-0340) the continuum, due to the severe line crowding in these metal- and with the NASA/ESA Hubble Space Telescope, obtained at the rich objects, iii) the presence of strong molecular (mainly ff Space Telescope Science Institute, operated by AURA Inc. under TiO) bands in the brightest, coolest stars, together with e ects contract to NASA. of α-element enhancements on the continuum absorption; iv) Tables 1, 2, 6 and Fig. 1 are only available in electronic form at the uncertainty of the effective temperature scale for stars as http://www.edpsciences.org hot as the Horizontal Branch (HB): temperatures derived by Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20041014 508 M. Zoccali et al.: Metal content of NGC 6528 imposing excitation equilibrium for the Fe I lines tend to be overestimated probably due to blends, and possibly to NLTE effects on Fe I in giants, whereas temperatures derived from colours show uncertainties due to reddening variations. A few stars in each of the two GCs have been observed in the past few years at high spectral resolution, but derived metal- licities show discrepancies, higher than the estimated uncer- tainty, thus revealing that there are systematic errors not prop- erly taken into account. In NGC 6553, two were observed with CASPEC, at the ESO 3.6 m Telescope, by Barbuy et al. (1999). They derived [Fe/H] = −0.55, [Na/Fe] ≈ [Al/Fe] ≈ [Ti/Fe] ≈ +0.5, [Mg/Fe] ≈ [Si/Fe] ≈ [Ca/Fe] ≈ +0.3and [s − elements/Fe] ∼< 0.0. A significantly higher metal- licity was obtained instead by Cohen et al. (1999), from Fig. 2. The SOFI near-IR (left) and HST optical (right)CMDof = − HIRES@Keck I observations of 5 HB stars: [Fe/H] 0.16, NGC 6528 with the target stars shown as big filled circles. [α/Fe] ≈ +0.2. More recently, Origlia et al. (2002) measured the metallicity of NGC 6553 from near-IR spectra obtained with NIRSPEC@Keck II, obtaining [Fe/H] = −0.3, with [α/Fe] =+0.3. Meléndez et al. (2003) used PHOENIX@Gemini-South to obtain spectra of R = 50 000 in the H band, and they derived [Fe/H] = −0.20 and [O/Fe] =+0.20. A summary of the previous metallicity determinations of NGC 6553 is given in Table 5 of Barbuy et al. (1999). For NGC 6528 the only high-resolution analysis published so far has been that of Carretta et al. (2001, hereafter C01), who observed four HB stars, with HIRES at Keck I. They obtained [Fe/H] =+0.07 and [M/H] =+0.17. Recently, Momany et al. (2003) determined the metallicity of NGC 6528 from RGB morphology indicators, in a (K, V − K) CMD, obtain- − − ing values in the range 0.43 < [Fe/H] < 0.22 depending on Fig. 3. NGC 6528: original and reddening corrected V vs. V −I Colour the adopted calibration. Magnitude Diagrams. In this work we present detailed abundance analysis of three stars in NGC 6528 using high resolution échelle of the ESO New Technology Telescope (NTT). The details spectra obtained with UVES at the ESO VLT-UT2 Kueyen about both optical and near-IR data are given in Momany et al. 8 m telescope, in an attempt to reduce the discrepancy be- (2003). tween previous measurements using higher spectral resolution Five isolated stars were selected from our sample for spec- (R ∼ 50 000) relative to previous studies (e.g. R ∼ 37 000 troscopic observations (Fig. 1). The target location on both the in C01 and R ∼ 20 000 in Barbuy et al. 1999). The analysis SOFI near-IR and the HST optical CMD is shown in Fig. 2. is restricted to relatively hot (Teff > 4000 K) stars, in order to Table 1 lists the coordinates, magnitudes and colours of the avoid the above mentioned problems with molecular bands. sample stars. The star’s identifications follow the notation by The paper is organized as follows. Spectroscopic observa- van den Bergh & Younger (1979), where the prefix I corre- tions are described in Sect. 2. Section 3 focuses on the determi- sponds to the internal ring. For each star the first line corre- nation of the stellar parameters: effective temperature and grav- sponds to the original magnitudes, while the second line cor- ity from photometry and spectroscopy. Equivalent width and responds to the magnitudes corrected for total (see Sect. 3.1) oscillator strenghts are discussed in Sect. 3. Iron abundance is and differential extinction. The latter correction has been car- derived in Sect. 4 and abundance ratios in Sect. 5. The results ried out following the method described in Piotto et al. (1999) are then discussed in Sect. 6. and Zoccali et al. (2001). This method assumes that the shift relative to the RGB fiducial mean locus, between different sub- 2. The data regions, is dominated by differential reddening. The observed field is divided into small subregions, and the reddening of each 2.1. Imaging of them is estimated by comparison with the fiducial mean lo- V and I photometry of the central region of NGC 6528 cus of NGC 6528 from Momany et al. (2003). The result of this was obtained using two sets of WFPC2 observations with procedure for the (V, V − I) CMD is shown in Fig. 3. the Hubble Space Telescope (HST), (GO5436: Ortolani et al. A posteriori, we identified some of our targets also in 1995; GO8696: Feltzing & Johnson 2002). Infrared J, H, Ks the field of view of a J, H set of NICMOS@HST data observations were obtained with the SOFI infrared camera (Ortolani et al. 2001). The spatial resolution of the NICMOS M. Zoccali et al.: Metal content of NGC 6528 509 3. Stellar parameters 3.1. Reddening A reddening of E(B − V) = 0.55 was given in Momany et al. (2003), however this was a mean value from determinations relative to NGC 6553 and 47 Tucanae, in different colours (see their Table 3), and those values are somewhat dependent on the metallicity and α-element assumptions in isochrone mod- els.