Astronomy & Astrophysics

Free access article

Issue		A&A Volume 376, Number 1, September II 2001
Page(s)		136 - 143
Section		Stellar clusters and associations
DOI		10.1051/0004-6361:20010976

A&A 376, 136-143 (2001)
DOI: 10.1051/0004-6361:20010976

Membership of 23 stars towards the bulge globular clusters NGC 6528 and NGC 6553

P. Coelho¹, B. Barbuy¹, M.-N. Perrin², T. Idiart¹, R. P. Schiavon³, S. Ortolani⁴ and E. Bica⁵

¹ Universidade de São Paulo, Dept. de Astronomia, CP 3386, São Paulo 01060-970, Brazil
    e-mail: pcoelho@usp.br; barbuy@iagusp.usp.br; thais@iagusp.usp.br
² Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
    e-mail: Marie-Noel.Perrin@obspm.fr
³ Observatório Nacional, rua General José Cristino 77, 20921-400 Rio de Janeiro, Brazil and Present address: UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
    e-mail: ripisc@ucolick.org
⁴ Università di Padova, Dept. di Astronomia, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
    e-mail: ortolani@pd.astro.it
⁵ Universidade Federal do Rio Grande do Sul, Dept. de Astronomia, CP 15051, Porto Alegre 91501-970, Brazil
    e-mail: bica@if.ufrgs.br

(Received 30 January 2001 / Accepted 8 June 2001 )

Abstract
Low resolution spectra of 23 stars towards the bulge globular clusters NGC 6528 and NGC 6553 are analysed. Radial velocities and atmospheric parameters are derived in order to check their membership in the clusters. Effective temperatures were obtained from photometric data for stars with $T_{\rm eff} > 3800$ K, whereas for cooler stars, they were derived from equivalent widths of TiO bands. Calibrations of W(TiO) as a function of stellar parameters based on a grid of synthetic spectra are presented. Metallicities were derived from a comparison of the observed spectra to a grid of synthetic spectra. The sample comprises evolutionary stages from the Red Giant Branch to the Horizontal Branch, with parameters in the range $3200 \leq T_{\rm eff} \leq 5000$ K and $-0.5 \leq \log g \leq 2.4$ . The mean metallicities obtained for NGC 6528 and NGC 6553 are [ Fe/H $] \approx -0.5$ and -0.7, in both cases with [ Mg/Fe] = +0.3; assuming the same overabundance for the $\alpha$ elements O, Mg, Si, S, Ca and Ti, this gives $[Z/Z_{\rm\odot}] = -0.25$ and -0.45. Membership verification by means of low resolution spectra is a crucial step in preparing targets for high resolution spectroscopy with 8 m class telescopes.

Key words: clusters: globular: NGC 6528, NGC 6553 -- stars: abundances

Offprint request: P. Coelho, pcoelho@usp.br

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