Theoretical models for classical cepheids. IV. Mean magnitudes and colors and the evaluation of distance, reddening, and metallicity

We discuss the metallicity effect on the theoretical visual and near-infrar ed period-luminosity (PL) and period-luminosity-color (PLC) relations of cl assical Cepheids, as based on nonlinear, nonlocal, and time-dependent conve ctive pulsating models at varying chemical composition. In view of the two usual methods of averaging (magnitude-weighted and intensity-weighted) obse rved magnitudes and colors over the full pulsation cycle, in the first part of the paper we briefly discuss the differences between static and mean qu antities. We show that the behavior of the synthetic mean magnitudes and co lors fully reproduces the observed trend of Galactic Cepheids, supporting t he validity of the model predictions. In the second part of the paper we sh ow how the estimates of the mean reddening, EB-V, and true distance modulus , mu(0), of a galaxy from Cepheid VK photometry depend on the adopted metal content, in the sense that larger metallicities drive the host galaxy to l ower extinctions and distances. Conversely, self-consistent estimates of th e Cepheid mean reddening, distance, and metallicity may be derived if three -filter data (in this paper BVK) are taken into account. By applying the th eoretical PL and PLC relations to available BVK data of Cepheids in the Mag ellanic Clouds, we eventually obtain Z similar to 0.008, EB-V similar to 0. 02 mag, and mu(0) similar to 18.63 mag for the LMC; and Z similar to 0.004, EB-V similar to 0.01 mag, and mu(0) similar to 19.16 for the SMC. The disc repancy between such reddenings and the current values based on BVI data (E B-V similar to 0.074 mag for the LMC and similar to 0.054 mag for the SMC) is briefly discussed.