Period–luminosity–color–radius relationships of Cepheids as a function of metallicity: evolutionary effects

Based on consistent evolutionary and pulsation calculations, we analyse the effect of metallicity and of different convection treatments in the stellar models on period– magnitude, –color and –radius relationships. In order to perform an accurate comparison with observations, we have computed grids of atmosphere models and synthetic spectra for different metallicities, covering the range of effective temperatures and gravities relevant for Cepheids. The models are compared to recent observations of galactic and Magellanic Clouds Cepheids. Unprecedented level of agreement is found between models and observations. We show that within the range of metallicity for the Galaxy and the Magellanic Clouds, a change of slope in the period– luminosity (PL) relationship is predicted at low periods, due to the reduction of the blue loop during core He burning. The minimum mass undergoing a blue loop and consequently the critical period at which this change of slope occurs depend on the metallicity Z and on the convection treatment in the stellar models. However, besides this change of slope, we do not find any significant effect of metallicity on period–magnitude relationships from V to K bands, and on period–color relationships in IR colors. We only find a detectable effect of Z on (B − V ) colors. These results are not affected by uncertainties inherent in current stellar models, mainly due to convection treatment.