逆行轨道与顺行轨道之比：对柯伊伯带双星形成理论的检验
The Ratio of Retrograde to Prograde Orbits: A Test for Kuiper Belt Binary Formation Theories
摘要：
随着柯伊伯带双星的发现，有广泛的分离和大致相等的质量，新的理论被提出来解释他们的形成。Goldreich和他的合作者提出了两种形成方案：在第一种方案中，由大量小天体产生的动力学摩擦使一个短暂的双星成为束缚（$L^2s$机制）;在第二种方案中，一个短暂的双星通过与第三个天体相遇而被束缚（$L^3$机制）。我们发现，这些不同的二元形成的情况下留下自己独特的签名，在相对丰富的逆行双星轨道。这个特征是由于稳定的逆行轨道，它存在于希尔球中比逆行轨道更远的地方。它提供了一个很好的机会，以区分不同的二元形成方案的观测。我们预测，如果二进制的形成进行，而亚希尔速度占上风，绝大多数的所有可比的质量比二进制逆行轨道。这种逆行双星轨道的优势是通过$L^2s$机制或任何其他以平滑和渐进的方式耗散能量的机制形成双星的结果。对于超希尔速度的双星形成过程中通过$L^3$机制，产生了大致相同数量的逆行和逆行双星。这些预测假设，由于动力摩擦和柯伊伯带的动力搅动，随后的轨道演化并没有改变双星轨道形成后的方向。
英文摘要：
With the discovery of Kuiper Belt binaries that have wide separations and roughly equal masses new theories were proposed to explain their formation. Two formation scenarios were suggested by Goldreich and collaborators: In the first, dynamical friction that is generated by a sea of small bodies enables a transient binary to become bound ($L^2s$ mechanism); in the second, a transient binary gets bound by an encounter with a third body ($L^3$ mechanism). We show that these different binary formation scenarios leave their own unique signatures in the relative abundance of prograde to retrograde binary orbits. This signature is due to stable retrograde orbits that exist much further out in the Hill sphere than prograde orbits. It provides an excellent opportunity to distinguish between the different binary formation scenarios observationally. We predict that if binary formation proceeded while sub-Hill velocities prevailed, the vast majority of all comparable mass ratio binaries have retrograde orbits. This dominance of retrograde binary orbits is a result of binary formation via the $L^2s$ mechanism, or any other mechanism that dissipates energy in a smooth and gradual manner. For super-Hill velocities binary formation proceeds via the $L^3$ mechanism which produces a roughly equal number of prograde and retrograde binaries. These predictions assume that subsequent orbital evolution due to dynamical friction and dynamical stirring of the Kuiper belt did not alter the sense of the binary orbit after formation.
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