（65）Cybele的平衡形状：原始的还是大撞击的遗迹？
The equilibrium shape of (65) Cybele: primordial or relic of a large impact?
摘要：
Cybele小行星构成了一个吸引人的原始物质库，与外太阳系有遗传联系，最大成员的物理特性可以很容易地通过大型望远镜获得。我们在2021年7月和8月利用（65）Cybele的明亮幻影获取了小行星的高角分辨率图像和光学光变曲线，我们的目标是分析其形状和整体性质。利用VLT/SPHERE获取的7组图像，结合光学光变曲线，采用ADAM、MPCD和SAGE算法重建小行星的形状。通过N体模拟研究了形状的起源。Cybele的体积当量直径为263+/-3km，体积密度为1.55+/-0.19g.cm-3。值得注意的是，它的形状和旋转状态与麦克劳林平衡图的形状和旋转状态密切兼容。由于没有与Cybele相关的碰撞族，而且该天体的体积密度高于其他大型P型小行星，这表明它从未经历过任何大的破坏性撞击，随后又迅速重新积聚。这意味着它现在的形状代表了原来的形状。然而，对假设族的长期动力学演化的数值积分表明，它因重力扰动和演化陀螺上的混乱扩散而分散。西伯莉与平衡星系非常接近的匹配，开启了外太阳系直径大于260公里的小天体都是在平衡状态下形成的可能性。然而，我们不能排除旧的碰撞是平衡形状的起源。西伯里本身被发现是动态不稳定的，这意味着它最近（<1Ga）被放置在它目前的轨道上，要么是通过从西伯里区域相对稳定的轨道缓慢扩散，要么是从行星交叉区域的不稳定JFC轨道。
英文摘要：
Cybele asteroids constitute an appealing reservoir of primitive material genetically linked to the outer Solar System, and the physical properties of the largest members can be readily accessed by large telescopes. We took advantage of the bright apparition of (65) Cybele in July and August 2021 to acquire high-angular-resolution images and optical light curves of the asteroid with which we aim to analyse its shape and bulk properties. 7 series of images acquired with VLT/SPHERE were combined with optical light curves to reconstruct the shape of the asteroid using the ADAM, MPCD, and SAGE algorithms. The origin of the shape was investigated by means of N-body simulations. Cybele has a volume-equivalent diameter of 263+/-3km and a bulk density of 1.55+/-0.19g.cm-3. Notably, its shape and rotation state are closely compatible with those of a Maclaurin equilibrium figure. The lack of a collisional family associated with Cybele and the higher bulk density of that body with respect to other large P-type asteroids suggest that it never experienced any large disruptive impact followed by rapid re-accumulation. This would imply that its present-day shape represents the original one. However, numerical integration of the long-term dynamical evolution of a hypothetical family shows that it is dispersed by gravitational perturbations and chaotic diffusion over Gyrs of evolution. The very close match between Cybele and an equilibrium figure opens up the possibility that D>260km small bodies from the outer Solar System all formed at equilibrium. However, we cannot rule out an old impact as the origin of the equilibrium shape. Cybele itself is found to be dynamically unstable, implying that it was recently (<1Ga) placed on its current orbit either through slow diffusion from a relatively stable orbit in the Cybele region or, less likely, from an unstable, JFC orbit in the planet-crossing region.
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