* Astronomy

Title: Formation of Kuiper Belt Binaries
Authors: Hilke E. Schlichting, Re'em Sari
(Version v2))

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 (L²s mechanism); in the second, a binary is formed by three body gravitational deflection (L³ mechanism). Here, we accurately calculate the L²s and L³ formation rates for sub-Hill velocities. While the L²s formation rate is close to previous order of magnitude estimates, the L³ formation rate is about a factor of 4 smaller. For sub-Hill KBO velocities (v << v_H) the ratio of the L³ to the L²s 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 L³ mechanism dominates over the L²s mechanism. Binary formation via the L³ 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 L³ mechanism, contrary to Lee et al. (2007). For the L²s 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 >~ 0.002) and is most likely not crucial for KBB formation.

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Title: Formation of Kuiper Belt Binaries
Authors: Hilke E. Schlichting, Re'em Sari

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 (L² _s mechanism); in the second, a binary is formed by three body gravitational deflection (L³ mechanism). Here, we accurately calculate the L² _s and L³ formation rates for sub-Hill velocities. While the L² _s formation rate is close to previous order of magnitude estimates, the L³ formation rate is about a factor of 4 smaller. For sub-Hill KBO velocities (v << v_H) the ratio of the L³ to the L² _s formation rate is 0.05(v/v_H) independent of the small bodies' velocity dispersion, surface density and mutual collisions. For Super-Hill velocities (v >> v_H) the L³ mechanism dominates over the L² _s mechanism. Binary formation via the L³ mechanism competes with binary destruction by passing bodies. Given sufficient time, a statistical equilibrium abundance of binaries forms. We show here 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 L³ mechanism, contrary to Lee et al. (2007). For the L² _s mechanism we find that the typical time, transient binaries must last, to form Kuiper Belt binaries (KBB) for a given strength of dynamical friction, D, increases only logarithmically with D. Longevity of transient binaries becomes important only for very weak dynamical friction (i.e. D >=  0.002) and is most likely not crucial for KBB formation.

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Unique Data Collected on Double Asteroid Antiope
Combining precise observations obtained by ESO's Very Large Telescope with those gathered by a network of smaller telescopes, astronomers have described in unprecedented detail the double asteroid Antiope, which is shown to be a pair of rubble-pile chunks of material, of about the same size, whirling around one another in a perpetual pas de deux. The two components are egg-shaped despite their very small sizes.
The asteroid (90) Antiope was discovered in 1866 by Robert Luther from Dusseldorf, Germany. The 90th asteroid ever discovered, its name comes from Greek mythology. In 2000, William Merline and his collaborators found that the asteroid was composed of two similarly-sized components, making it a truly 'double' asteroid, one of the very first of this kind in the main belt of asteroids that lies between the orbits of Mars and Jupiter.

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VLT observations of the double asteroid (90) Antiope during 2004. The adaptive optics NACO instrument was used, allowing the astronomers to perfectly distinguish the two components and so, precisely determine the orbit. The two objects are separated by 171 km, and they perform their celestial dance in 16.5 hours. The adaptive optics observations could, however, never resolve the shape of the individual components as they are too small.
Credit ESO

Title: Binaries in the Kuiper Belt
Authors: Keith S. Noll, William M. Grundy, Eugene I. Chiang, Jean-Luc Margot, Susan D. Kern

Binaries have played a crucial role many times in the history of modern astronomy and are doing so again in the rapidly evolving exploration of the Kuiper Belt. The large fraction of transneptunian objects that are binary or multiple, 48 such systems are now known, has been an unanticipated windfall. Separations and relative magnitudes measured in discovery images give important information on the statistical properties of the binary population that can be related to competing models of binary formation. Orbits, derived for 13 systems, provide a determination of the system mass. Masses can be used to derive densities and albedos when an independent size measurement is available. Angular momenta and relative sizes of the majority of binaries are consistent with formation by dynamical capture. The small satellites of the largest transneptunian objects, in contrast, are more likely formed from collisions. Correlations of the fraction of binaries with different dynamical populations or with other physical variables have the potential to constrain models of the origin and evolution of the transneptunian population as a whole. Other means of studying binaries have only begun to be exploited, including lightcurve, colour, and spectral data. Because of the several channels for obtaining unique physical information, it is already clear that binaries will emerge as one of the most useful tools for unravelling the many complexities of transneptunian space.

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