Gravoturbulent Planetesimal Formation in the Early Solar System

我们研究了早期太阳星云中嵌入的由球粒和基质物质组成的引力不稳定云的星子的形成。星子前体物质（球粒或球粒大小的颗粒加上基质物质）在早期太阳星云中由于凝固、破碎和漂移而演化。在我们的项目中，我们将前驱物质丰度的预测与原始球粒陨石的组成联系起来，在星子引力湍流形成情景的框架下。在这种情况下，来自一系列进料区的前体物质被集中在太阳星云的非层流气体结构中（纬向流、涡旋和不稳定流），达到引力不稳定密度，例如，超过局部罗氏密度。在新的资助期内，我们将研究这些物质浓度的最终崩塌和可能的破碎，将我们的前体物质演化模型与原始球粒陨石、小行星和彗星的物理和化学性质联系起来。基于我们对太阳星云内部气体和尘埃物质演化的模拟，我们得出了前驱物质的局部丰度和大小分布随时间的变化。从地带流、涡旋和流不稳定性的磁流体动力学模拟中，我们知道这些前体物质集中的强度有多大，以及在过密度中会发生多大的大小分离。我们利用这些过密云，在局部高分辨率的水文和尘埃模拟中模拟它们的引力坍缩，基本上可以降低到固体密度（在空间分辨率上只有2-3个数量级），也就是说，我们可以在空间上通过多达1000个网格单元来解析形成的星子。因此，我们可以测量，例如，球粒的比例与基质材料的质量比例。在这个过程中，球粒将是最具流动性的粒子，驱动着坍塌的动力学，而基质物质则被动地被气体拖着走，并且只会在一个迄今无法预测的水平上被纳入星子中。通过改变前体云内的初始质量、体积和大小分布，我们可以找到产生某种球粒状物质的最佳点。我们可以将球粒陨石的性质与它的前体性质以及它在星云内的形成时间和位置联系起来。人们已经可以推测，在星云的演化过程中，不同的位置和时间将产生不同大小的球粒陨石，其内部大小分布也不同。通过对球粒陨石中物质的尺寸分布及其化学变化、小行星带和柯伊伯带天体的尺寸分布及其观测到的双星性等观测数据进行人工星子种群特性的检验，可以完善星子建筑材料前体演化模型，并检验星子形成的物理条件。

We investigate the formation of planetesimals from a gravitationally unstable cloud of chondrules and matrix material embedded in the early solar nebula. The planetesimal precursor material (chondrules or chondrule-sized grains plus matrix material) evolves due to coagulation, fragmentation, and drift in the early solar nebula. In our project we connect predictions on the abundance of precursor material to the composition of primitive chondrites in the framework of gravoturbulent formation scenario of planetesimals. In this scenario precursor material from a range of feeding zones gets concentrated in non-laminar gas structures of the solar nebula (zonal flows, vortices, and streaming instability) to gravitational unstable densities, e.g., exceeding the local Roche density. In the new funding period we will study the final collapse and possible fragmentation of these material concentrations to connect our models of precursor material evolution to the physical and chemical properties of primitive chondrites, asteroids, and cometesimals. Based on our simulations of gas and dust material evolution inside the solar nebula, we derive a time-dependent local abundance and size distribution of precursor material. We know from our magnetohydrodynamical simulations of zonal flows, vortices, and the streaming instability how strong this precursor material gets concentrated and what size segregation will occur in the overdensities. We use these overdense clouds and simulate their gravitational collapse in local high resolution hydro and dust simulations basically down to the solid density (which is only 2-3 orders of magnitude in spatial resolution), i.e., we can spatially resolve the forming planetesimals by up to 1000 grid cells in volume. Thus, we can measure, for instance, the fraction of chondrules vs. the mass fraction in matrix material. Chondrules will in this process be the most mobile particles, driving the dynamics of the collapse, whereas the matrix material is passively dragged along by the gas, and will only at a so far unpredicted level get incorporated into the planetesimal. By varying the initial mass, volume, and size distribution within the precursor cloud we can find the sweet spot that produces a certain kind of chondritic material. We can link the properties of a chondrite to its precursor properties and its formation time and location inside the nebula. One can already speculate that during the evolution of the nebula different locations and times will produce different sized chondrites with varying internal size distributions. By testing the properties of our artificial population of planetesimals to the observed data on size distributions of the material in chondrites, its chemical variations, and size distributions of asteroid belt and Kuiper belt objects and their observed binarity, we can improve our model of precursor evolution of planetesimal building material and also test the physical conditions for planetesimal formation.