  Context: Classical Cepheids can be adopted to trace the chemical evolution of
the Galactic disk since their distances can be estimated with very high
accuracy. Aims: Homogeneous iron abundance measurements for 33 Galactic
Cepheids located in the outer disk together with accurate distance
determinations based on near-infrared photometry are adopted to constrain the
Galactic iron gradient beyond 10 kpc. Methods: Iron abundances were determined
using high resolution Cepheid spectra collected with three different
observational instruments: ESPaDOnS@CFHT, Narval@TBL and<EMAIL_ADDRESS>ESO/MPG
telescope. Cepheid distances were estimated using near-infrared (J,H,K-band)
period-luminosity relations and data from SAAO and the 2MASS catalog. Results:
The least squares solution over the entire data set indicates that the iron
gradient in the Galactic disk presents a slope of -0.052+/-0.003 dex/kpc in the
5-17 kpc range. However, the change of the iron abundance across the disk seems
to be better described by a linear regime inside the solar circle and a
flattening of the gradient toward the outer disk (beyond 10 kpc). In the latter
region the iron gradient presents a shallower slope, i.e. -0.012+/-0.014
dex/kpc. In the outer disk (10-12 kpc) we also found that Cepheids present an
increase in the spread in iron abundance. Current evidence indicates that the
spread in metallicity depends on the Galactocentric longitude. Finally, current
data do not support the hypothesis of a discontinuity in the iron gradient at
Galactocentric distances of 10-12 kpc. Conclusions: The occurrence of a spread
in iron abundance as a function of the Galactocentric longitude indicates that
linear radial gradients should be cautiously treated to constrain the chemical
evolution across the disk.
