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

尘埃聚集体的体积密度是原行星盘中尘埃碰撞生长的一个重要因素。聚集体的密度改变了与圆盘气体的耦合，从而控制了聚集体在圆盘中的运动。碰撞结果还取决于聚集体的密度。为了研究原行星盘中生长的尘埃聚集体的密度演变，我们对由许多亚微米大小的冰粒组成的聚集体的连续碰撞进行了三维n体模拟。在当前的序列碰撞模拟中，我们使用在前一次碰撞中得到的聚合体作为每次碰撞的初始聚合体。通过重复聚集体碰撞的n体计算，研究了聚集体碰撞的密度演化。在粉尘生长初期，团聚体相互粘附而不重组，团聚体密度降低。在后期，当冲击能量超过临界能量时，聚集体逐渐被压缩。压缩聚集体的分形维数为2.5。由于这种小的分形维数，即使在这个压缩阶段，它们的密度仍然很低。我们还推导了一个描述生长聚集体密度演化的方程。将此方程应用于原行星盘中的尘埃生长，我们发现尘埃聚集体的密度极低（< 0.1 kg m-3）。在本研究的模拟中，我们只考虑正面碰撞。斜碰撞的影响会进一步降低骨料密度。

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.