THE COMBINED EFFECTS OF COLD-NEBULA DRAG AND MEAN-MOTION RESONANCES

A model for Epstein drag forces in the Solar System's primordial dust cloud is proposed; this model is consistent with the asteroid belt for ming in a cold nebula. Small bodies are assumed to move in exact Keple rian orbits in the absence of drag; the orbital average of the collisi onal effects leads to circularization of orbits, for any density profi le of the nebula. An increase in density near Jupiter's orbit is assum ed. In the presence of such a density gradient but in the absence of a massive perturber, orbital radii will increase or decrease according to whether the relative outward gradient is larger or smaller than a c ritical value. Given both a density gradient and a massive Jupiter, an alysis of the planar restricted three-body problem so modified leads t o a condition on the relative density gradient for the (initially smal l) eccentricity of an asteroid near a given resonance with the perturb er to increase or decrease. These analytical results on the evolution of asteroid eccentricities in the combined presence of mean-motion res onances and solar-nebula drag effects are found to be in good agreemen t with those obtained by a seminumerical averaging scheme. Based on th ese analytical and seminumerical results, we formulate a self-consiste nt hypothesis for the general pattern of gaps and groups in the astero id belt. (C) 1998 Academic Press.