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

尘埃聚集体的体积密度是影响原行星盘中尘埃碰撞增长的重要因素。聚集体的密度改变了与圆盘气体的耦合，这决定了聚集体在圆盘中的运动。碰撞的结果也取决于聚集体的密度。我们进行了三维N体模拟的连续碰撞的聚集体组成的一些亚微米大小的冰粒，以调查的密度演化的尘埃聚集体生长在原行星盘。在连续碰撞的本模拟中，作为每次碰撞的初始聚集体，我们使用在前一次碰撞中获得的合成聚集体。通过重复的N体凝聚碰撞计算，我们研究了凝聚碰撞的密度演化。在尘埃生长的早期阶段，聚集体彼此粘附而不重组，并且这些聚集体的密度降低。在冲击能量超过临界能量的后期阶段，聚集体被逐渐压缩。压缩聚集体具有2.5的分形维数。由于这种小的分形维数，它们的密度即使在该压缩阶段也保持非常低。我们还推导出一个方程描述的密度演化的增长聚集体。将这个方程应用于原行星盘中的尘埃生长，我们发现尘埃聚集体具有极低的密度（< 0.1 kg m-3）。在本研究的模拟中，我们只考虑正面碰撞。斜碰撞的影响将进一步降低聚集体密度。

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.