Physical Conditions in the Primitive Solar Nebula

Abstract

Models of the primitive solar nebula have been constructed. A model is required to be in centrifugal equilibrium radially in the plane of the disk and in hydrostatic equilibrium perpendicular to the plane of the disk. The distribution of angular momentum per unit mass in the initial model is that appropriate to a fragment of a collapsing interstellar gas cloud.

For a reasonable distribution of the angular momentum per unit mass, there are two solutions with centrifugal equilibrium. One solution has a very flat surface density, which is unstable against many perturbations, and presumably forms a double star system. The other solution is much more axially condensed and is taken to be an approximate model of the primitive solar nebula. This model is formed as a result of a dynamical collapse process which produces initial interior temperatures in the vicinity of 10⁴K. The collapse causes initial turbulence, and there is a resulting redistribution of angular momentum as a result of turbulent viscosity. This has been taken into account in an approximate way and the resulting redistribution of surface density has been calculated.

The nebula cools at constant central pressure. In the range 2000–5000 K the opacity is small and the nebula is in radiative equilibrium. At lower temperatures the opacity is greatly increased due to molecules like H₂O and to condensed particles, mainly iron and silicate grains. This produces thermally driven convection out to a nebular radius of a few astronomical units. In this region there is renewed turbulent viscosity, which leads to outward transport of angular momentum and inward net transport of mass to form the sun.

The accumulation of small particles into larger bodies always takes place in the presence of gas and is assisted by turbulence and acceleration by electric fields. Lightning flashes may play a role in the formation of chondrules. Continued convective transport through a range of temperatures must play an important role in the chemistry of the accumulating material. The relation of these processes to the range of conditions presented in the nebular model is discussed.