THRESHOLDS AND THE CHEMICAL EVOLUTION OF GALACTIC DISKS .3. EFFECT OFTHE 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 rotatio n curve were assumed to be constant in time and produced by an invisib le massive halo. If the self-gravitation of the disc is included, the epicyclic frequency varies with time. In this paper, we have taken acc ount of the gradual build-up of the rotation curve, making the approxi mation that the baryonic matter of the disc only contributes to the ci rcular velocity once it has reached the disc, and not when it is infal ling. However, even in models in which the disc mass is the major cont ributor to the rotation curve at late times, the disc properties are v ery little different from those of models in which the influence of th e disc mass is ignored.