(216)Kleopatra，一颗低密度临界旋转的M-型小行星
(216) Kleopatra, a low density critically rotating M-type asteroid
摘要：
上下文最近对M/Xe型三重小行星系统（216）$^{-3}$的三维形状的估计表明其密度为5。g.cm这样的高密度意味着高金属含量和低孔隙率，这不容易与其特有的哑铃形状相协调。目标。鉴于VLT/SPHERE/ZIMPOL相机前所未有的角分辨率，我们的目标是以高精度限制克娄巴特拉的质量和形状，从而限制其密度。方法.我们将2017年和2018年记录的Kleopatra的新VLT/SPHERE观测与档案数据以及光曲线，掩星和延迟多普勒图像相结合，使用两种不同的算法（ADAM，MPCD）推导出其3D形状模型。此外，N体动力学模型使我们能够检索两颗卫星的轨道要素，如随附论文所述。结果克娄巴特拉的形状非常接近平衡哑铃图与两个叶片和一个粗脖子。它的体积当量直径（118.75 $\pm$ 1.40）km和质量（2.97 $\pm$ 0.32）10 $^{18}$ kg意味着体积密度为（3.38 $\pm$ 0.50）g cm $^{-3}$。如此低的密度对于一个假定的富含金属的身体表明一个实质性的孔隙度内的原生。这种多孔结构及其接近平衡的形状与包括巨大撞击后再累积的地层情景是相容的。克娄巴特拉目前的自转周期和哑铃形状意味着它正处于临界自转状态。沿着天体赤道的低有效重力，连同卫星的赤道轨道和可能的碎石堆结构，打开了卫星通过质量脱落形成的可能性。结论.克娄巴特拉是一个令人费解的多重系统，由于独特的特点，主要的。它值得在未来特别关注，随着极大望远镜和可能的专门空间任务。
英文摘要：
Context. The recent estimates of the 3D shape of the M/Xe-type triple asteroid system (216) Kleopatra indicated a density of 5 g.cm$^{-3}$. Such a high density implies a high metal content and a low porosity which is not easy to reconcile with its peculiar dumbbell shape. Aims. Given the unprecedented angular resolution of the VLT/SPHERE/ZIMPOL camera, we aim to constrain the mass and the shape of Kleopatra with high accuracy, hence its density. Methods. We combined our new VLT/SPHERE observations of Kleopatra recorded in 2017 and 2018 with archival data, as well as lightcurve, occultation, and delay-Doppler images, to derive its 3D shape model using two different algorithms (ADAM, MPCD). Furthermore, an N-body dynamical model allowed us to retrieve the orbital elements of the two moons as explained in the accompanying paper. Results. The shape of Kleopatra is very close to an equilibrium dumbbell figure with two lobes and a thick neck. Its volume equivalent diameter (118.75$\pm$1.40) km and mass (2.97$\pm$0.32) 10$^{18}$ kg imply a bulk density of (3.38$\pm$0.50) g cm$^{-3}$. Such a low density for a supposedly metal-rich body indicates a substantial porosity within the primary. This porous structure along with its near-equilibrium shape is compatible with a formation scenario including a giant impact followed by reaccumulation. Kleopatra's current rotation period and dumbbell shape imply that it is in a critically rotating state. The low effective gravity along the equator of the body, together with the equatorial orbits of the moons and possibly rubble-pile structure, opens the possibility that the moons formed via mass shedding. Conclusions. Kleopatra is a puzzling multiple system due to the unique characteristics of the primary. It deserves particular attention in the future, with the Extremely Large Telescopes and possibly a dedicated space mission.
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