PARTICLE-GAS DYNAMICS IN THE PROTOPLANETARY NEBULA

J.N. Cuzzi, R.C. Hogan, and A.R. Dobrovolskis

"Primitive"or unmelted asteroids, from which the planets were built, are represented in the meteorite record as a vast data set that has had little context for interpretation. The accretion of these primitive bodies from small grains and mm-sized, melted ''chondrules" almost certainly occurred in the presence of gas. Study of this stage is complicated by the feedback effects of the gas on the particles, and vice versa. Ames' efforts focus on numerical modeling of particle-gas interactions in turbulent flows, and understanding meteorite properties in the light of theoretical models.

Ames' "turbulent concentration" theory (TC) , introduced several years ago, shows how particles of a specific size/density combination are concentrated by orders of magnitude in weak nebula turbulence. The theory makes specific predictions as to the relative abundance distribution of the concentrated particles. Predictions of the shape of the size distribution are in very good agreement with observed particle size distributions in primitive chondrites, thus revealing the fingerprints of TC. We developed a multifractal theory to predict the magnitude of turbulent concentration at much higher Reynolds numbers than achievable numerically, but the concentration factor can be so large that the local particle mass density can exceed that of the gas, and the feedback effect of the particle phase on damping the gas turblence must be considered before further modeling efforts can proceed. To better understand the effects of heavy mass loading on turbulence and TC, we are developing a cascade model of the process which is capable of reproducing the way concentrations of particles emerge as energy flows down the turbulent cascade, or inertial range. The cascade model is parametrized by partition functions or "multipliers" which are only statistically defined , but whose probability distribution function can be fit to our numerical results for mass-loaded turbulent fluids. That is to say, the multipliers appropriate for densely particle-enriched regions where the turbulent kinetic energy and/or vorticity might be damped, could be different from the multipliers in "normal" regions where mass loading is negligible. We are now determining the dependence of these multipliers on the local gas and/or particle density properties, making extensive use of new runs of a scalar field particle code (rather than the previous Lagarangian particle code) on the Ames Origins 2000 facility.

This year, Ames researchers also developed a scenario to help explain a new phenomenon found in chondritic meteorites by collaborators at Stanford and the University of Hawaii. The observation is an abundant class of Iron-Nickel metal grains with chemical and crystallographic properties that define their growth and cooling times simultaneously. The scenario developed visualizes a very hot, early, perhaps inner stage of the protoplanetary nebula, rather different from the environment in which more familiar chondrites form. In this dense, hot region, strong convection plumes rise towards the surface of the nebula, cooling and condensing small metal and silicate particles much as raindrops or hailstones condense in upwelling thunderstorm plumes on Earth. Some fraction of these objects are dispersed outwards to cooler regions before being downdrafted again to their destruction. While the theory is adequate to explain some properties of these unique meteorites, it is clear that deeply puzzling aspects remain unexplained.