Planetesimals are compact astrophysical objects roughly 1–1000 km in size, massive enough to be held together by gravity. They can grow by accreting material to become full-size planets. Planetesimals themselves are thought to form by complex physical processes from small grains in protoplanetary disks. The streaming instability (SI) model states that millimeter/centimeter-sized particles (pebbles) are aerodynamically collected into self-gravitating clouds that then directly collapse into planetesimals. Here we analyze ATHENA simulations of the SI to characterize the initial properties (e.g., rotation) of pebble clouds. Their gravitational collapse is followed with the PKDGRAV N-body code, which has been modified to realistically account for pebble collisions. We find that pebble clouds rapidly collapse into short-lived disk structures from which planetesimals form. The planetesimal properties depend on the cloud’s scaled angular momentum, , where L and M are the angular momentum and mass, R _H is the Hill radius, and Ω is the orbital frequency. Low-l pebble clouds produce tight (or contact) binaries and single planetesimals. Compact high-l clouds give birth to binary planetesimals with attributes that closely resemble the equally sized binaries found in the Kuiper Belt. Significantly, the SI-triggered gravitational collapse can explain the angular momentum distribution of known equally sized binaries—a result pending verification from studies with improved resolution. About 10% of collapse simulations produce hierarchical systems with two or more large moons. These systems should be found in the Kuiper Belt when observations reach the threshold sensitivity.

星子是大小约 1 至 1000 公里的紧凑型天体物理物体，其质量足以通过重力聚集在一起。它们可以通过吸积物质成长为全尺寸的行星。星子本身被认为是由原行星盘中的小颗粒通过复杂的物理过程形成的。流动不稳定性 (SI) 模型指出，毫米/厘米大小的粒子（鹅卵石）通过空气动力学收集到自引力云中，然后直接坍缩成星子。在这里，我们分析了 SI 的ATHENA模拟，以表征卵石云的初始属性（例如，旋转）。它们的引力坍缩伴随着PKDGRAV N-body 代码，已经过修改以真实地解释卵石碰撞。我们发现卵石云迅速坍缩成短暂的盘状结构，小行星从中形成。星子属性取决于云的缩放角动量，其中L和M是角动量和质量，R _H是希尔半径，Ω 是轨道频率。低升卵石云产生紧（或接触）的二进制文件和单个微行星。紧凑型高升云产生了双星子，其属性与柯伊伯带中发现的同样大小的双星非常相似。重要的是，SI 触发的引力坍缩可以解释已知大小相等的双星的角动量分布——这一结果有待提高分辨率的研究进行验证。大约 10% 的坍塌模拟产生具有两个或更多大卫星的分层系统。当观测达到阈值灵敏度时，应该在柯伊伯带中找到这些系统。