Detailed chemical composition of Galactic Cepheids^*

A determination of the Galactic abundance gradient in the 8–12 kpc region

¹ Université de Picardie Jules Verne, Faculté des Sciences, 33 rue Saint-Leu, 80039 Amiens Cedex 1, France
² Observatoire de Paris-Meudon, GEPI, 92195 Meudon Cedex, France e-mail: Bertrand.Lemasle@etud.u-picardie.fr
³ European Southern Observatory (ESO), 3107 Alonso de Cordova, Vitacura, Casilla 19001, Santiago 19, Chile
⁴ INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
⁵ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, 85748 Garching bei Muenchen, Germany

Received: 11 September 2006
Accepted: 10 January 2007

Abstract

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 v_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.