Pulsational constraints to the metallicity gradient in the Galactic disk

BVI data of Galactic Cepheids observed in the solar neighborhood and in the outer disk are analyzed on the basis of nonlinear, non-local and time-dependent convective pulsating models computed for different assumptions on the chemical composition. We show that the theoretical color-color, period-luminosity, and period-luminosity-color relations, as predicted by the models with varying metal content, provide the way to simultaneously estimate the true distance modulus mu (0) and the metal content Z of each individual variable. On this basis, we estimate that the metallicity distribution of Cepheids with Galactocentric distances d from similar to6 to similar to 19 kpc can be represented by a linear gradient of 0.05 +/- 0.01 dex kpc(-1), which is close to previous empirical determinations based on Cepheids, Planetary Nebulae and HII regions. However, our results are not excluding a different scenario, such as a two-zone model with a discontinuity in metallicity near d = 10 kpc (see Twarog et al. 1997).