Period-colour and amplitude-colour relations in classical Cepheid variables - II. The Galactic Cepheid models

In this paper, we construct full amplitude non-linear hydrodynamical models of fundamental mode Galactic Cepheids and analyse the resulting theoretical period-colour (PC) and amplitude-colour (AC) relations at maximum, mean and minimum light. These theoretical relations match the general form of the observed relations well. This agreement is, to some extent, independent of the mass-luminosity (ML) relations used, pulsation code, numerical techniques, details of the input physics and methods to convert theoretical quantities, such as bolometric luminosity and temperature, to observational quantities, such as V-band magnitudes or (V-I) colours. We show that the PC and AC properties of fundamental mode Galactic Cepheids with periods such that log(P) > 0.8 can be explained by a simple application of the Stefan-Boltzmann law and the interaction of the photosphere with the hydrogen ionization front (HIF). We discuss the implications of our results for explaining the behaviour of Galactic Cepheid PC and period-luminosity (PL) relations at mean light.