Theoretical Models for Classical Cepheids : II . Period - Luminosity , Period - Color and Period - Luminosity - Color Relations

We present and discuss theoretical predictions concerning the pulsational properties of Classical Cepheids. Masses and luminosities provided by stellar evolutionary calculations are used as input parameters of nonlinear, nonlocal and time–dependent convective pulsating models and accurate determinations of both the blue and red edge of the instability strip are derived, together with theoretical light curves for a suitable grid of models. The computations have been performed for three different chemical compositions (Y=0.25, Z=0.004; Y=0.25, Z=0.008; Y=0.28, Z=0.02), taken as representative of Cepheids in the Magellanic Clouds (MCs) and in the Galaxy. Bolometric light curves have been transformed into visual and near-infrared magnitudes and the intensity-weighted mean magnitudes of the pulsator over a full pulsation cycle (< MV > and < MK >, respectively) are obtained. We derive that either in the logP–< MV > and in the logP–< MK > planes the predicted edges of instability strip are in excellent agreement with the observed distribution of Galactic and Magellanic Cepheids, providing a preliminary estimate of the distance to these galaxies. Moreover, we show that the models are in agreement with several empirical Period-Luminosity (PL) relations given in the literature, even though the theoretical distribution in logP–< MV > plane is better represented by a quadratic PL relation. We also show that both the zero-point and the slope of the predicted PL relations are significantly dependent on metallicity, with the amplitude of the metallicity effect decreasing at the longer wavelength. At variance with several empirical suggestions appeared in the literature, we find that at fixed period the metal–rich pulsators should be fainter than the metal–poor ones. Tight Period-Luminosity-Color (PLC) relations are derived for both visual