THE DYNAMIC EVOLUTION OF THE PROTOSOLAR NEBULA

We calculate the global evolution of prostostellar nebulae for a variety of initial disk masses and angular momenta. Although we approximate the viscous stress by an "alpha" model, we calibrate the constant alpha in terms of the properties of turbulent thermal convection. As the evolution proceeds, the nebula viscously spreads and is partially accreted by the protostar. The consequent decrease of the surface density renders portions of the disk optically thin in the vertical direction. When the Rosseland mean optical depth decreases below a critical value of order unity, we assume that the disk is unable locally to sustain thermal convection and that the turbulent viscosity and energy dissipation cease. We further assume that once convection subsides, no other sources of turbulent stress take its place. We calculate the resulting disk evolutionary properties under these assumptions and make comparisons with the solar system. We find that (1) moderately viscous disks with alpha almost-equal-to 10(-2) evolve to marginally (Rosseland) optically thin conditions in approximately 6 x 10(5) yr; (2) the resulting disk radii are primarily sensitive to the initial nebular angular momentum and insensitive to the intial disk mass; (3) significant amounts of mass (approximately 0.1 M.) can remain behind in a nonturbulent disk if the initial disk angular momentum is high; and (4) the heating of the disk surface inside r less-than-or-similar-to 1 AU by protosolar illumination can halt thermal convection after approximately 5 x 10(5) yr. We speculate that the termination of convective turbulent viscosity in the outer solar system marks the inception of the giant planet-building epoch by allowing grains to coagulate and sediment to the nebular midplane.