FORMATION OF KUIPER BELT BINARIES BY GRAVITATIONAL COLLAPSE

A large fraction of similar to 100 km class low-inclination objects in the classical Kuiper Belt (KB) are binaries with comparable masses and a wide separation of components. A favored model for their formation is that they were captured during the coagulation growth of bodies in the early KB. However, recent studies have suggested that large, similar to 100 km objects can rapidly form in the protoplanetary disks when swarms of locally concentrated solids collapse under their own gravity. Here, we examine the possibility that KB binaries formed during gravitational collapse when the excess of angular momentum prevented the agglomeration of available mass into a solitary object. We find that this new mechanism provides a robust path toward the formation of KB binaries with observed properties, and can explain wide systems such as 2001 QW(322) and multiples such as (47171) 1999 TC36. Notably, the gravitational collapse is capable of producing similar to 100% binary fraction for a wide range of the swarm's initial angular momentum values. The binary components have similar masses (similar to 80% have a secondary-over-primary radius ratio >0.7) and their separation ranges from similar to 1000 to similar to 100,000 km. The binary orbits have eccentricities from e = 0 to similar to 1, with the majority havinge < 0.6. The binary orbit inclinations with respect to the initial angular momentum of the swarm range from i = 0 to similar to 90 degrees, with most cases having i < 50 degrees. The total binary mass represents a characteristic fraction of the collapsing swarm's total initial mass, M-tot, suggesting M-tot equivalent to that of a radius similar to 100-250 km compact object. Our binary formation mechanism also implies that the primary and secondary components in each binary pair should have identical bulk composition, which is consistent with the current photometric data. We discuss the applicability of our results to the Pluto-Charon, Orcus-Vanth, (617) Patroclus-Menoetius, and (90) Antiope binary systems.