行星前天体间的吸积：失控增长的多面性
Accretion among preplanetary bodies: the many faces of runaway growth
摘要：
当行星前天体的体积达到~1 km或更大时，它们的吸积速率由于引力聚焦（GF）而增强。我们发展了一个新的数值模型来计算引力增强生长阶段的碰撞演化。我们验证我们的方法对现有的N体和统计代码。利用数值模型，我们探讨了从单个星子半径R 0的初始种群开始的失控生长和寡头生长吸积阶段的特征。在模型中，初始随机速度分散（源自它们的偏心率）开始低于微行星体的逃逸速度，系统经历失控的增长。我们发现，在失控的增长阶段的尺寸分布保持连续，但演变成幂律在高质量端，与以前的研究一致。此外，我们发现，最大的机构从所有的质量箱吸积，一个简单的两个组件的近似是不适用的，在这个阶段。然而，随着增长，逃逸体激起了它所吸积的小行星群体的随机运动。最终，这种反馈停止了快速增长，系统进入寡头政治，来自邻近地区的竞争对手在质量上迎头赶上。与以前的估计相比，我们发现，该系统离开失控的增长阶段在一个更大的半径。此外，我们评估的相关性小，单一大小的片段的生长过程。在经典模型中，物体的初始速度分散很小，在失控增长过程中这些不起关键作用;然而，在模型中，由于外部搅拌其随机运动，其特点是初始相对速度大，可能出现碎片占主导地位的情况。
英文摘要：
(abridged) When preplanetary bodies reach proportions of ~1 km or larger in size, their accretion rate is enhanced due to gravitational focusing (GF). We have developed a new numerical model to calculate the collisional evolution of the gravitationally-enhanced growth stage. We validate our approach against existing N-body and statistical codes. Using the numerical model, we explore the characteristics of the runaway growth and the oligarchic growth accretion phases starting from an initial population of single planetesimal radius R_0. In models where the initial random velocity dispersion (as derived from their eccentricity) starts out below the escape speed of the planetesimal bodies, the system experiences runaway growth. We find that during the runaway growth phase the size distribution remains continuous but evolves into a power-law at the high mass end, consistent with previous studies. Furthermore, we find that the largest body accretes from all mass bins; a simple two component approximation is inapplicable during this stage. However, with growth the runaway body stirs up the random motions of the planetesimal population from which it is accreting. Ultimately, this feedback stops the fast growth and the system passes into oligarchy, where competitor bodies from neighboring zones catch up in terms of mass. Compared to previous estimates, we find that the system leaves the runaway growth phase at a somewhat larger radius. Furthermore, we assess the relevance of small, single-size fragments on the growth process. In classical models, where the initial velocity dispersion of bodies is small, these do not play a critical role during the runaway growth; however, in models that are characterized by large initial relative velocities due to external stirring of their random motions, a situation can emerge where fragments dominate the accretion.
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