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
  (abridged) We have observed almost 1/3 of the globular clusters in the Milky
Way, targeting distant and/or highly reddened objects, besides a few reference
clusters. A large sample of red giant stars was observed with FORS2@VLT/ESO at
R ~ 2,000. The method for derivation of stellar parameters is presented with
application to six reference clusters. We aim at deriving the stellar
parameters effective temperature, gravity, metallicity and alpha-element
enhancement, as well as radial velocity, for membership confirmation of
individual stars in each cluster. We analyse the spectra collected for the
reference globular clusters NGC 6528, NGC 6553, M 71, NGC 6558, NGC 6426 and
Terzan 8. They cover the full range of globular cluster metallicities, and are
located in the bulge, disc and halo. Full spectrum fitting techniques are
applied, by comparing each target spectrum with a stellar library in the
optical region at 4560-5860 A. We employed the library of observed spectra
MILES, and the synthetic library by Coelho et al. (2005). Validation of the
method is achieved through recovery of 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 MILES and Coelho et al. libraries. 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 abundances for NGC 6426
from spectroscopy for the first time. The method proved to be reliable for red
giant stars observed with resolution R ~ 2,000, yielding results compatible
with high-resolution spectroscopy. The derived alpha-element abundances show
[A/Fe] vs. [Fe/H] consistent with that of field stars at the same
metallicities.
