Utilitarian Models of the Solar Nebula

Abstract

Models of the primitive solar nebula based on a combination of theory, observations of T Tauri stars, and global conservation laws are presented. The models describe the motions of nebular gas, mixing of interstellar material during the formation of the nebula, and evolution of thermal structure in terms of several characteristic parameters. The parameters describe key aspects of the protosolar cloud (its rotation rate and collapse rate) and the nebula (its mass relative to the Sun, decay time, and density distribution). For most applications, the models are heuristic rather than predictive. Their purpose is to provide a realistic context for the interpretation of solar system data, and to distinguish those nebula characteristics that can be specified with confidence, independently of the assumptions of particular models, from those that are poorly constrained. It is demonstrated that nebular gas typically experienced large radial excursions during the evolution of the nebula and that both inward and outward mean radial velocities on the order of meters per second occurred in the terrestrial planet region, with inward velocities predominant for most of the evolution. However, the time history of disk size, surface density, and radial velocities are sensitive to the total angular momentum of the protosolar cloud, which cannot be constrained by purely theoretical considerations. It is shown that a certain amount of "formational" mixing of interstellar material was an inevitable consequence of nebular mass and angular momentum transport during protostellar collapse, regardless of the specific transport mechanisms involved. Even if the protosolar cloud was initially homogeneous, this mixing was important because it had the effect of mingling presolar material that had experienced different degrees of thermal processing during collapse and passage through the accretion shock. Nebular thermal structure is less sensitive to poorly constrained parameters than is dynamical history. A simple criterion is derived for the condition that silicate grains are evaporated at midplane, and it is argued that this condition was probably fulfilled early in nebular history. Cooling of a hot nebula due to coagulation of dust and consequent local reduction of optical depth is examined, and it is shown how such a process leads naturally to an enrichment of rock-forming elements in the gas phase. © 1994 Academic Press. All rights reserved.