Context: Classical Cepheids are excellent tracers of intermediate-mass stars, since their distances can be estimated with very high accuracy. In particular, they can be adopted to trace the chemical evolution of the Galactic disk.
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 FEROS@2.2m 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 textrm {dex kpc}^-1 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 textrm {dex kpc}^-1. 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.