Formation and Evolution of the Solar Nebula

Abstract

The protostellar phase of stellar evolution is of considerable importance in determining whether a solar nebula forms from the collapse of an interstellar cloud, what the physical properties of the nebula are at the onset of its evolution, what the dominant mechanisms for angular momentum transport will be during the subsequent evolution, and whether conditions are favorable for the formation of planets. The initial mass distribution and angular momentum distribution in the core of a molecular cloud determine whether a binary system or a single star is formed. A relatively slowly rotating and centrally condensed cloud is likely to collapse to a disk-like structure out of which planets can form. The above parameters then determine the temperature and density structure of the disk and the characteristics of the resulting planetary system.

There has been considerable recent interest in two- and three-dimensional numerical hydrodynamical calculations with radiative transfer, applied to the inner regions of collapsing, rotating protostellar clouds of about 1 M_⨀. The calculations start at a density that is high enough so that the gas is decoupled from the magnetic field. Three-dimensional calculations show amplification of initial non-axisymmetric perturbations during collapse. If such perturbations are relatively small, angular momentum transport by gravitational torques is slow enough so that an axisymmetric approximation is sufficiently accurate to give useful results. Under the further assumption that angular momentum transport by turbulent viscosity is not important on a collapse time, calculations can be performed under the assumption of conservation of angular momentum of each mass element. Once the disk forms, however, transport processes must be included. This paper concentrates on the formation phase and its influence on the later evolutionary phases.

With a suitable choice of initial angular momentum, the size of the disk is similar to that of our planetary system. The disk forms as a relatively thick, warm equilibrium structure, with a shock wave separating it from the surrounding infalling gas. The calculations give temperature and density distributions throughout the infalling cloud as a function of time. From these, frequency-dependent radiative transfer calculations produce infrared spectra and isophote maps at selected viewing angles. The theoretical spectra may be compared with observations of suspected protostellar sources, under the hypothesis that the observed objects actually represent precursors to “solar” nebulae.