FORS2/VLT survey of Milky Way globular clusters

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.