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Origin of asteroid rotation rates in catastrophic impacts

机译：灾难性影响中小行星旋转速率的起源

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摘要

The rotation rates of asteroids, which are deduced from periodic fluctuations in their brightnesses, are controlled by mutual collisions. The link between asteroid spin and collision history is usually made with reference to impact experiments on centimetre-scale targets, where material strength governs the impact response. Recent work, however, indicates that for objects of the size of most observed asteroids (> or = 1 km in diameter), gravity rather than intrinsic strength controls the dynamic response to collisions. Here we explore this idea by modelling the effect of impacts on large gravitating bodies. We find that the fraction of a projectile's angular momentum that is retained by a target asteroid is both lower and more variable than expected from laboratory experiments, with spin evolution being dominated by 'catastrophic' collisions that eject approximately 50 per cent of the target's mass. The remnant of an initially non-rotating silicate asteroid that suffers such a collision rotates at a rate of approximately 2.9 per day, which is close to the observed mean asteroid rotation rate of approximately 2.5 d-1. Moreover, our calculations suggest that the observed trend in the mean spin frequency for different classes of asteroids (2.2 d-1 for C-type asteroids, 2.5 d-1 for S-type, and 4.0 d-1 for M-type) is due to increasing mean density, rather than increasing material strength.
机译：小行星的旋转速度是由相互碰撞控制的，小行星的旋转速度是由其亮度的周期性波动得出的。小行星自旋与碰撞历史之间的联系通常是参考以厘米为单位的目标进行碰撞实验，在该实验中，材料强度决定了碰撞响应。但是，最近的研究表明，对于大多数观察到的小行星（直径大于或等于1 km）大小的物体，重力而不是内在强度控制着对碰撞的动态响应。在这里，我们通过对撞击在大型引力体上的效果进行建模来探索这种想法。我们发现，目标小行星保留的弹丸角动量的分数比实验室实验预期的要低，并且变化更大，自旋演变受``灾难性''碰撞的支配，该碰撞弹出了目标质量的50％。最初发生这种碰撞的非旋转硅酸盐小行星的残余物以每天约2.9的速率旋转，这接近于观察到的约2.5 d-1的平均小行星旋转速率。此外，我们的计算表明，不同类别的小行星（C型小行星为2.2 d-1，S型小行星为2.5 d-1，M型小行星为4.0 d-1）的平均自旋频率观察到的趋势是由于增加了平均密度，而不是增加了材料强度。

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