Distribution of spin axes and senses of rotation for 20 large asteroids
Distribution of spin axes and senses of rotation for 20 large asteroids
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DOI：
10.1016/0019-1035(86)90072-2
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发表时间：
1986-10
期刊：
影响因子：
3.2
通讯作者：
P. Magnusson
中科院分区：
文献类型：
--
作者：
P. Magnusson
介绍了20颗大小行星的极点测定。这是第一次确定11个天体的旋转方向，其余9个小行星中有两个的旋转方向与以前的结果相反。自旋轴是相当各向同性分布的，具有统计上不确定的倾向于顺时针旋转。自转角速度矢量向黄道北极方向的分量的平均值为ωz ω =（0.8 ± 0.5）转/日。这表明，对于大的小行星来说，碰撞并没有完全随机化原先占优势的旋光体（本样本的中位直径约为200公里）。为了得到尽可能可靠的结果，将两种根本不同的极点测定方法结合起来。第一种是基于三轴椭球模型的振幅-幅值法。天球扫描与审判极点和一个被选择的最佳拟合获得半经验的幅度-方位-相位和幅度-方位-相位的关系。用这种方法也得到了真实小行星形状的三轴近似。第二种方法是利用观测到的旋转会合周期的变化来推导旋转轴和旋转方向。在光变曲线中选择一个定义明确的“标准特征”，并假设它保持在一个固定的旋转相位。一个有效的算法，用于找到正确的旋转周期数之间的意见，在不同的幻影。这使得可以彼此识别在不同显现期间观察到的极值（假设例如，在一个相对位置处的主最大值在其它方位角处保持主最大值）。用这种方法还可以区分不明确的自转周期。4灶神星在每个自转周期中有一个极大值和一个极小值。7个Iris和15个Eunomia的自转周期的长期变化分别小于3 × 10− 4和2 × 10− 4秒/年。
Pole determinations for 20 large asteroids are presented. This is the first determination of the sense of rotation for 11 of the objects, and a sense of rotation opposite to previous results is obtained for two of the remaining nine asteroids. The spin axes are fairly isotropically distributed, with a statistically uncertain preference for prograde rotation. The mean of the component of the spin angular velocity vectors toward the north ecliptic pole is 〈ωz〉 = (0.8 ± 0.5) rev/day. This suggests that for large asteroids an original predominance of prograde rotators has not completely been randomized by collisions (the median diameter in the present sample is approximately 200 km). Two fundamentally different pole determination methods were combined in order to get as reliable results as possible. The first is an Amplitude-Magnitude method based on triaxial ellipsoidal models. The celestial sphere is scanned with trial poles and the one is chosen for which the best fit is obtained with semiempirical amplitude-aspect-phase and magnitude-aspect-phase relationships. Triaxial approximations to the true asteroidal shapes are also obtained with this method. The second method uses the variation of the observed synodic period of rotation to derive the axis and sense of rotation. A well-defined “standard feature” in the lightcurves is selected and is assumed to remain at a fixed rotational phase. An efficient algorithm for finding the correct number of rotational cycles between observations during different apparitions is used. This makes it possible to identify extrema observed during different apparitions with each other (it is not safe to assume that, e.g., the primary maximum at one opposition remains primary at other aspect angles). Discrimination between ambiguous rotation periods can also be made with this method. 4 Vesta is shown to have one maximum and one minimum per rotational cycle. The secular variations of the period of rotation for 7 Iris and 15 Eunomia are less than 3 × 10−4and 2 × 10−4sec/year, respectively.