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

太阳引力垂直分量引起的尘埃沉降是太阳星云中行星吸积前的一个重要过程。我们研究了太阳星云中尘埃粒子的沉降和凝聚，同时考虑到尘埃聚集体的分形形态。我们发现，尘埃聚集体的分形形态仅在早期阶段保持不变，并很快恢复到分形维数D = 3，因此，尘埃聚集体的沉降时间和最终尺寸与Nakagawa等人（1986）的结果非常相似，后者没有考虑尘埃聚集体的分形形态，而总是假定尘埃粒子为球形。然而，沉降的特征与D“*”3的情况不同，在沉降的早期阶段，粉尘聚集体没有明显的沉降，而是逐渐长大，而在D = 3的恢复阶段，由于粉尘聚集体的内部密度迅速增加，沉降加速，粉尘生长减速。这就是为什么在D 3和D“*”3的情况下，建立时间和最终半径没有明显差异的原因<less than or equal>。至于引力不稳定性是否真的发生在尘埃层中，我们还没有得到最终的结论。所以我们将继续我们的

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