THE BROWNIAN-MOTION OF DUST PARTICLES IN THE SOLAR NEBULA - AN EXPERIMENTAL APPROACH TO THE PROBLEM OF PREPLANETARY DUST AGGREGATION

Laboratory experiments were performed to study the Brownian motion of mu m-sized dust grains and small aggregates under pre-planetary nebula conditions, i.e., in a thin gas atmosphere (Epstein drag regime), in the ballistic limit, and under microgravity conditions. The results of these experiments, i.e., the grain diffusivities, are in quantitative agreement with theoretical predictions for single spherical grains. D eviations from particle sphericity, i.e., in our case aggregates consi sting of monodisperse spherical grains, cause only minor deviations be tween the Epstein drag formula for spheres and our experimental result s for equal particle cross section. Thus, we find a quantitative agree ment of our measurements with the Epstein relation D proportional to 1 /sigma(a) between grain diffusivity and geometrical (aerodynamic) cros s section. The results of our investigations can be used for the calcu lation of the gas-grain stopping time tau(f) = epsilon (m/sigma(a)) (1 /rho(g) nu(g)) which, in turn, is an important grain characteristic fo r the calculation of pre-planetary dust aggregation. Here, m and sigma (a) are the mass and the aerodynamic, i.e., geometric cross section of the grain, rho(g) and nu(m) are the mass density of the gas and the m ean thermal velocity of the gas molecules, and epsilon is a proportion ality factor which we determined to be epsilon = 0.68 +/- 0.10. The ga s-grain stopping time describes the strength of grain coupling to a gi ven gas motion and its value determines relative velocties and, hence, collision frequencies between dust grains due to sedimentation, drag- induced orbital decay, and gas turbulence in the solar nebula. (C) 199 6 Academic Press, Inc.