Thresholds and the chemical evolution of galactic discs-III. Effects of the disc gravitational field

In previous papers, we have considered the evolution of galactic discs with a star formation threshold depending on the epicyclic frequency in the disc. In these papers, the disc gravitational field and rotation curve were assumed to be constant in time and produced by an invisible massive halo. If the self-gravitation of the disc is included, the epicyclic frequency varies with time. In this paper, we have taken account of the gradual build-up of the rotation curve, making the approximation that the baryonic matter of the disc only contributes to the circular velocity once it has reached the disc, and not when it is infalling. However, even in models in which the disc mass is the major contributor to the rotation curve at late times, the disc properties are very little different from those of models in which the influence of the disc mass is ignored.