arXiv 0709.31072007-09astro-ph

柯伊伯带双星的形成

Formation of Kuiper Belt Binaries

Hilke E. Schlichting, Re'em Sari

备注：: 20 pages, 3 figures, Accepted for publication in ApJ, correction of minor typos
DOI：: 10.1086/524930

摘要：

柯伊伯带天体（KBO）中有相当一部分存在于具有宽间距和大致相等质量的双星中，这一发现激发了各种解释其形成的新理论。Goldreich等人（2002）提出了两种形成方案：第一种方案是形成一个短暂的双星，它在来自小天体海洋的动力学摩擦的帮助下结合在一起（L^2s机制）;第二种方案是通过三体引力偏转形成一个双星（L^3机制）。在这里，我们精确地计算了亚希尔速度下的L^2s和L^3形成速率。虽然L^2 s的形成速率接近于先前的数量级估计，但L^3的形成速率大约小4倍。对于亚希尔KBO速度（v << v_H），L^3与L^2s的形成速率之比为0.05（v/v_H），与小天体的速度弥散、表面密度或相互碰撞无关。对于超希尔速度（v >> v_H），L^3机制优于L^2s机制。通过L^3机制形成的双星与通过天体的双星相抗衡。如果有足够的时间，双星系统会形成一个统计上的平衡丰度。我们发现，长寿命的瞬变双星的频率随着系统的寿命呈指数下降，并且这种瞬变双星对于通过L^3机制形成的双星并不重要，与Lee等人（2007）相反。对于L^2s机制，我们发现，对于给定的动力学摩擦强度D，典型的时间瞬变双星必须持续存在，才能形成柯伊伯带双星（KBB），而D只随D的增加而增加。短暂双星的寿命只有在非常弱的动力学摩擦（即D \lesssim 0.002）时才变得重要，而对KBB的形成可能并不重要。

英文摘要：

The discovery that a substantial fraction of Kuiper Belt objects (KBOs) exists in binaries with wide separations and roughly equal masses, has motivated a variety of new theories explaining their formation. Goldreich et al. (2002) proposed two formation scenarios: In the first, a transient binary is formed, which becomes bound with the aid of dynamical friction from the sea of small bodies (L^2s mechanism); in the second, a binary is formed by three body gravitational deflection (L^3 mechanism). Here, we accurately calculate the L^2s and L^3 formation rates for sub-Hill velocities. While the L^2s formation rate is close to previous order of magnitude estimates, the L^3 formation rate is about a factor of 4 smaller. For sub-Hill KBO velocities (v << v_H) the ratio of the L^3 to the L^2s formation rate is 0.05 (v/v_H) independent of the small bodies' velocity dispersion, their surface density or their mutual collisions. For Super-Hill velocities (v >> v_H) the L^3 mechanism dominates over the L^2s mechanism. Binary formation via the L^3 mechanism competes with binary destruction by passing bodies. Given sufficient time, a statistical equilibrium abundance of binaries forms. We show that the frequency of long-lived transient binaries drops exponentially with the system's lifetime and that such transient binaries are not important for binary formation via the L^3 mechanism, contrary to Lee et al. (2007). For the L^2s mechanism we find that the typical time, transient binaries must last, to form Kuiper Belt binaries (KBBs) for a given strength of dynamical friction, D, increases only logarithmically with D. Longevity of transient binaries only becomes important for very weak dynamical friction (i.e. D \lesssim 0.002) and is most likely not crucial for KBB formation.

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