ダストアグリゲイトの衝突数値シミュレーション : ダストの圧縮・破壊と構造進化

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2章惑星形成と氷微粒子惑星形成はダスト (μm以下のサイズの粒子が多数付着しアグリゲイトになっている) 同士が衝突合体成長していくことから始まるが，その詳細は不明な点が多い．本研究では，衝突によるダストアグリゲイトの構造進化過程を明らかにするべく，３次元アグリゲイトの正面衝突の数値シミュレーションを行った．結果として，アグリゲイトは衝突によってあまり圧縮されない (フラクタル次元は～2.5) ことが示され，圧縮過程のスケーリング則および「状態方程式」が得られた．また，氷粒子からなるアグリゲイトは，数十m/sの高速衝突でも成長できることが示された．以上のことから，ダストの衝突合体によって低密度な微惑星が形成されることが示唆される．We study collisions between dust aggregates to construct a model of their structural evolution in protoplanetary disks. We carry out three-dimensional simulations of aggregate collisions and examine their compression and disruption processes. We take clusters of ballistic cluster-cluster aggregation (BCCA) formed by a hit-and-stick process as initial structures and study their head-on collisions with realistic binding forces. Our numerical results indicate that the energy criteria for compression and disruption of BCCA clusters are consistent with previous twodimensional simulations. For aggregate compression due to collision, we succeed in obtaining a scaling law that the gyration radius of the resultant aggregate is proportional to Eimp-0.10, where Eimp is the impact energy. Furthermore, we derive an “equation of state” of aggregates which reproduces the scaling law for compression. This equation of state is useful to describe the density evolution of dust aggregates during their growth. The aggregates are not compressed well by their mutual collisions and the fractal dimension of the compressed aggregates is at most ～2.5. The aggregates composed of ice particles are possible to grow even if the impact velocities are as high as a few tens of m/s. These numerical results suggest that very fluffy planetesimals can be formed in protoplanetary disks by direct collisional growth of dust.
2008-03-31

ダストアグリゲイトの衝突数値シミュレーション : ダストの圧縮・破壊と構造進化

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