UTILITARIAN MODELS OF THE SOLAR NEBULA

Models of the primitive solar nebula based on a combination of theory, observations of T Tauri stars, and global conservation laws are prese nted. The models describe the motions of nebular gas, mixing of inters tellar material during the formation of the nebula, and evolution of t hermal structure in terms of several characteristic parameters. The pa rameters describe key aspects of the protosolar cloud (its rotation ra te and collapse rate) and the nebula (its mass relative to the Sun, de cay time, and density distribution). For most applications, the models are heuristic rather than predictive. Their purpose is to provide a r ealistic context for the interpretation of solar system data, and to d istinguish those nebula characteristics that can be specified with con fidence independently of the assumptions of particular models, from th ose that are poorly constrained. It is demonstrated that nebular gas t ypically experienced large radial excursions during the evolution of t he nebula and that both inward and outward mean radial velocities on t he order of meters per second occurred in the terrestrial planet regio n, with inward velocities predominant for most of the evolution. Howev er, the time history of disk size, surface density, and radial velocit ies are sensitive to the total angular momentum of the protosolar clou d, which cannot be constrained by purely theoretical considerations. I t is shown that a certain amount of ''formational'' mixing of interste llar material was an inevitable consequence of nebular mass and angula r momentum transport during protostellar collapse, regardless of the s pecific transport mechanisms involved. Even if the protosolar cloud wa s initially homogeneous, this mixing was important because it had the effect of mingling presolar material that had experienced different de grees of thermal processing during collapse and passage through the ac cretion shock. Nebular thermal structure is less sensitive to poorly c onstrained parameters than is dynamical history. A simple criterion is derived for the condition that silicate grains are evaporated at midp lane, and it is argued that this condition was probably fulfilled earl y in nebular history. Cooling of a hot nebula due to coagulation of du st 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. (C) 1994 Academic Press, Inc.