Volume 573, January 2015
|Number of page(s)||23|
|Section||Galactic structure, stellar clusters and populations|
|Published online||10 December 2014|
FORS2/VLT survey of Milky Way globular clusters
I. Description of the method for derivation of metal abundances in the optical and application to NGC 6528, NGC 6553, M 71, NGC 6558, NGC 6426, and Terzan 8⋆,⋆⋆
Universidade de São Paulo,
Dept. de Astronomia, Rua do Matão
2 European Southern Observatory, 3107 Alonso de Cordova, Santiago, Chile
3 INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
4 Research School of Astronomy & Astrophysics, Australian National University, Mount Stromlo Observatory, via Cotter Road, Weston Creek, ACT 2611, Australia
5 Università di Padova, Dipartimento di Astronomia, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
6 Instituto de Astrofisica, Facultad de Fisica, Pontificia Universidad Catolica de Chile, Casilla 306 Santiago 22, Chile
7 GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
Received: 14 April 2014
Accepted: 12 September 2014
Context. We have observed almost one third of the globular clusters in the Milky Way, targeting distant and/or highly reddened objects, as well as a few reference clusters. A large sample of red giant stars was observed with FORS2 at VLT/ESO at R ~ 2000. The method for deriving stellar parameters is presented with application to six reference clusters.
Aims. We aim at deriving the stellar parameters effective temperature, gravity, metallicity, and alpha-element enhancement as well as the radial velocity to confirm the membership of individual stars in each cluster. We analyse the spectra collected for the reference globular clusters NGC 6528 ([Fe/H] ~−0.1), NGC 6553 ([Fe/H] ~−0.2), M 71 ([Fe/H] ~−0.8), NGC 6558 ([Fe/H] ~−1.0), NGC 6426 ([Fe/H] ~−2.1), and Terzan 8 ([Fe/H] ~−2.2). They cover the full range of globular cluster metallicities, and are located in the bulge, disc, and halo.
Methods. Full spectrum-fitting techniques were applied by comparing each target spectrum with a stellar library in the optical region at 4560−5860 Å. We employed the library of observed spectra MILES, and the Coelho synthetic library. We validated the method by recovering the known atmospheric parameters for 49 well-studied stars that cover a wide range in the parameter space. We adopted as final stellar parameters (effective temperatures, gravities, metallicities) the average of results using the observed and synthetic spectral libraries.
Results. We identified 4 member stars in NGC 6528, 13 in NGC 6553, 10 in M 71, 5 in NGC 6558, 5 in NGC 6426, and 12 in Terzan 8. Radial velocities, Teff, log(g), [Fe/H], and alpha-element enhancements were derived. We derived ⟨ vhelio ⟩ = −242 ± 11 km s-1, [Fe/H] = − 2.39 ± 0.04, [Mg/Fe] = 0.38 ± 0.06 for NGC 6426 from spectroscopy for the first time.
Conclusions. The method proved to be reliable for red giant stars observed with resolution R ~ 2000, yielding results compatible with high-resolution spectroscopy. The derived α-element abundances show [α/Fe] vs. [Fe/H] consistent with that of field stars at the same metallicities.
Key words: stars: abundances / stars: kinematics and dynamics / stars: Population II / globular clusters: general / Galaxy: stellar content
Based on observations collected at the European Southern Observatory/Paranal, Chile, under programmes 077.D-0775(A) and 089.D-0493(B).
Figures 14−18, Tables 3, 4, and 6 are available in electronic form at http://www.aanda.org
© ESO, 2014
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