FROM DUST TO PLANETESIMALS: AN IMPROVED MODEL FOR COLLISIONAL GROWTH IN PROTOPLANETARY DISKS
FROM DUST TO PLANETESIMALS: AN IMPROVED MODEL FOR COLLISIONAL GROWTH IN PROTOPLANETARY DISKS
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DOI：
10.1088/0004-637x/764/2/146
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发表时间：
2012-08
期刊：
影响因子：
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通讯作者：
P. Garaud;F. Meru;M. Galvagni;C. Olczak
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文献类型：
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作者：
P. Garaud;F. Meru;M. Galvagni;C. Olczak
行星的形成发生在原行星盘富含气体和尘埃的环境中。对这些物体的观测表明，原始亚微米大小的颗粒生长成更大的聚集体发生在圆盘的最早进化阶段。然而，使用Smoluchowski方程来描述碰撞凝聚和破碎的颗粒生长的理论模型迄今未能产生大颗粒，同时保持大量的小颗粒。这通常归因于存在两个障碍，由于碰撞粒子的反弹和破碎而阻碍生长。在本文中，我们表明，这些障碍的重要性已被人为膨胀，通过使用简化的模型，不考虑随机性质的气体盘内的粒子运动。我们提出了一种新的方法，其中两个粒子之间的相对速度所描述的概率分布函数，模型确定性运动（从垂直沉降，径向漂移，方位角漂移）和随机运动（从布朗运动和湍流）。考虑到这两个因素，可能会得出与最近其他研究中所考虑的完全不同的结果。我们证明了两个“成分”的颗粒生长的重要影响：适当的实施速度分布函数，克服了弹跳障碍，并结合在高质量比碰撞的质量传递，提高了增长的更大的颗粒超出破碎障碍。我们模拟的一个强有力的结果是两个粒子群（小和大）的出现，可能同时解释了原行星盘中的一些长期存在的问题，包括靠近中心星星的星子形成，在盘中存在毫米到厘米大小的粒子，以及微米大小的颗粒持续数百万年。
Planet formation occurs within the gas- and dust-rich environments of protoplanetary disks. Observations of these objects show that the growth of primordial submicron-sized particles into larger aggregates occurs at the earliest evolutionary stages of the disks. However, theoretical models of particle growth that use the Smoluchowski equation to describe collisional coagulation and fragmentation have so far failed to produce large particles while maintaining a significant population of small grains. This has generally been attributed to the existence of two barriers impeding growth due to bouncing and fragmentation of colliding particles. In this paper, we demonstrate that the importance of these barriers has been artificially inflated through the use of simplified models that do not take into account the stochastic nature of the particle motions within the gas disk. We present a new approach in which the relative velocities between two particles are described by a probability distribution function that models both deterministic motion (from the vertical settling, radial drift, and azimuthal drift) and stochastic motion (from Brownian motion and turbulence). Taking both into account can give quite different results to what has been considered recently in other studies. We demonstrate the vital effect of two “ingredients” for particle growth: the proper implementation of a velocity distribution function that overcomes the bouncing barrier and, in combination with mass transfer in high-mass-ratio collisions, boosts the growth of larger particles beyond the fragmentation barrier. A robust result of our simulations is the emergence of two particle populations (small and large), potentially explaining simultaneously a number of longstanding problems in protoplanetary disks, including planetesimal formation close to the central star, the presence of millimeter- to centimeter-sized particles far out in the disk, and the persistence of μm-sized grains for millions of years.