Theoretical Studies on Evolution of Dust Size and Structure in Protoplanetary Disks

尘埃团聚体的体积密度是原行星盘中尘埃碰撞增长的一个重要因素。团聚体的密度改变了与盘状气体的耦合，从而控制了团聚体在盘中的运动。碰撞的结果也取决于聚集密度。我们对由许多亚微米大小的冰粒组成的集合体的顺序碰撞进行了三维N体模拟，以研究在原行星盘中生长的尘埃集合体的密度演化。在当前的顺序碰撞模拟中，作为每次碰撞的初始聚集，我们使用在上一次碰撞中获得的结果聚集。通过重复聚集体碰撞的N体计算，我们考察了聚集体碰撞的密度演化。在粉尘生长的早期阶段，团聚体相互粘连而不重组，这些团聚体的密度下降。在撞击能量超过临界能量的较后阶段，聚集体被逐渐压缩。压缩集料的分维值为2.5。由于这种小的分维，它们的密度即使在这个压缩阶段也保持非常低的密度。我们还推导了描述生长集合体密度演化的方程。将这个方程应用于原行星盘中尘埃的增长，我们发现尘埃聚集体具有极低的密度(&lt；0.1 kg m-3)。在本研究的模拟中，我们只考虑迎头碰撞。斜碰撞的影响会进一步降低骨料的密度。

The bulk density of dust aggregates is an important factor in collisional growth of dust in protoplanetary disks. The density of aggregates changes the coupling with the disk gas, which governs the motion of aggregates in disks. Collisional outcomes also depend on the aggregate density. We perform three-dimensional N-body simulations of sequential collisions of aggregates composed of a number of sub-micron-sized icy particles in order to investigate the density evolution of dust aggregates growing in protoplanetary disks. In the present simulation of sequential collisions, as the initial aggregates at each collision, we use the resultant aggregate obtained at the previous collision. By repeating N-body calculations of aggregate collisions, we examined the density evolution of aggregate collisions. At an early stage of dust growth, aggregates stick to each other without restructuring, and the density of these aggregates decreases. At a later stage, in which the impact energy exceeds a critical energy, aggregates are gradually compressed. The compressed aggregates have a fractal dimension of 2.5. Because of this small fractal dimension, their density remains very low even at this compression stage. We also derive an equation describing the density evolution of growing aggregates. Applying this equation to dust growth in protoplanetary disks, we find that dust aggregates have an extremely low density (< 0.1 kg m-3). In the simulation of the present study, we consider only head-on collisions. The effect of oblique collisions would further reduce the aggregate density.