NEW POSSIBILITY OF PLANETESIMAL FORMATION THROUGH NO GRAVITATIONAL FRAGMENTATION OF A DUST LAYER

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Settling of dust particles caused by vertical component of the solar gravity is an important process in a solar nebula in advance of planetary accretion. We examined the settling and coagulation of dust particles in the solar nebula, taking into account fractal-like morphology of dust aggregates. We found that dust aggregates can keep fractal morphology just for early stages and soon recover the fractal dimension D of three and, hence, the settling time and the final sizes of dust aggregates are quite similar to those obtained by Nakagawa et al. (1986) who did not consider the fractal-like morphology of dust aggregates and always assumed spherical particles. However, the feature of settling is different from the case of D "*" 3 ; in early stages dust aggregates grow up to larger size without appreciable settling, while in the recovery phase toward D = 3 settling accelerates and dust growth decelerates because of rapid increase of the internal density of dust aggregates. This is the reason why there is no appreciable difference in settling time and the final radii between the cases of D <less than or equal> 3 and D "*" 3. As for the question whether the gravitational instability really occurs in the dust layer or not, we have not got the final conclusion yet. So we will continue our