朝向微行星：原行星星云中的致密球粒团块
Towards planetesimals: dense chondrule clumps in the protoplanetary nebula
摘要：
我们概述了一个场景，该场景追踪了从自由漂浮的星云颗粒到类地行星区域最初10- 100公里大小的天体的直接路径，产生了具有与原始陨石母体相匹配的属性的微行星。我们称之为“原始吸积”。该方案借鉴了以前的工作的元素，并介绍了一个新的关键门槛的星子形成。我们假设星云是弱湍流，这导致空气动力学大小排序的粒子具有类似于在陨石中观察到的性质的密集浓度。这些致密区域或团块所占据的星云体积分数服从概率分布，作为其密度的函数，并且致密浓度的粒子质量密度是气体的100倍。然而，即使是这些致密的团块也会被气体压力阻止经历传统意义上的引力不稳定性（在动力学时间尺度上）。在这种停滞发展的状态下，它们很容易受到不同轨道星云气体的冲压压力的破坏。然而，自重力可以保护足够大和密集的团块免受冲压压力的破坏，允许它们携带的颗粒缓慢但无情地向它们的中心沉积，产生10-100公里的“沙堆”星子。星云中的局部径向压力波动，以及不同运动的致密团块之间的相互作用，也将在未来的研究中发挥作用。这种情况很容易从陨石母体扩展到整个太阳系的初级吸积。
英文摘要：
We outline a scenario which traces a direct path from freely-floating nebula particles to the first 10-100km-sized bodies in the terrestrial planet region, producing planetesimals which have properties matching those of primitive meteorite parent bodies. We call this "primary accretion". The scenario draws on elements of previous work, and introduces a new critical threshold for planetesimal formation. We presume the nebula to be weakly turbulent, which leads to dense concentrations of aerodynamically size-sorted particles having properties like those observed in chondrites. The fractional volume of the nebula occupied by these dense zones or clumps obeys a probability distribution as a function of their density, and the densest concentrations have particle mass density 100 times that of the gas. However, even these densest clumps are prevented by gas pressure from undergoing gravitational instability in the traditional sense (on a dynamical timescale). While in this state of arrested development, they are susceptible to disruption by the ram pressure of the differentially orbiting nebula gas. However, self-gravity can preserve sufficiently large and dense clumps from ram pressure disruption, allowing their entrained particles to sediment gently but inexorably towards their centers, producing 10-100 km "sandpile" planetesimals. Localized radial pressure fluctuations in the nebula, and interactions between differentially moving dense clumps, will also play a role that must be allowed for in future studies. The scenario is readily extended from meteorite parent bodies to primary accretion throughout the solar system.
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