Long term gas drag effect on the structure of the asteroid belt and its implications for the solar nebula

We show that if the nebula gas was preserved for similar to 10(6-7) yr after formation of the giant planets, asteroids would experience migration due to the hydrodynamic drag as well as to giant planetary perturbations. These combined effects can lead to the present spatial distribution of the main asteroid belt. In particular, gas drag is necessary for the depletion of asteroids in regions outside 3.2 A.U. Asteroids at Jupiter's 2:1 resonance are strongly perturbed by its gravity. The enhanced eccentricity induces a rapid orbital migration and a sharp drop-off in the asteroid distribution. The implication of our model is that nebula gas with several Jupiter masses was retained in the vicinity of the asteroid belt for a few times 10(6) yr after the formation of Jupiter and the effective nebula viscosity during that stage corresponds to alpha similar or equal to 10(-4) in the alpha prescription. (C) 1996 American Astronomical Society.