Detailed chemical composition of Galactic Cepheids -: A determination of the Galactic abundance gradient in the 8-12 kpc region

Aims. The recent introduction of high-resolution/large spectral-range spectrographs has provided the opportunity to investigate the chemical composition of classical Cepheids in detail. This paper focusses on new abundance determinations for iron and 6 light metals (O, Na, Mg, Al, Si, Ca) in 30 Galactic Cepheids. We also give a new estimate of the Galactic radial abundance gradient. Methods. The stellar effective temperatures were determined using the method of line depth ratios, and the surface gravity and the microturbulent velocity upsilon(t) by imposing the ionization balance between Fe I and Fe II with the help of curves of growth. Abundances were calculated with classical LTE atmosphere models. Results. Abundances were obtained with rms accuracies of about 0.05-0.10 dex for Fe, and 0.05-0.20 dex for the other elements. Cepheids in our sample have solar-like abundances, and current measurements agree quite well with previous determinations. We computed single zone Galactic radial abundance gradients for the 8-12 kpc region and found a slope for iron of -0.061 dex kpc(-1).