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
Abstract
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.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- January 2015
- DOI:
- 10.1051/0004-6361/201423996
- arXiv:
- arXiv:1409.4374
- Bibcode:
- 2015A&A...573A..13D
- Keywords:
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- stars: abundances;
- stars: kinematics and dynamics;
- stars: Population II;
- globular clusters: general;
- Galaxy: stellar content;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Instrumentation and Methods for Astrophysics;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 22 pages, 21 figures, accepted for publication in A&