Formation of Kuiper Belt binaries by gravitational collapse

A large fraction of ∼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, ≳ 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 QW₃₂₂ and multiples such as (47171) 1999 TC₃₆. Notably, the gravitational collapse is capable of producing ∼100% binary fraction for a wide range of the swarm's initial angular momentum values. The binary components have similar masses (∼80% have a secondary-over-primary radius ratio <0.7) and their separation ranges from ∼1000 to ∼ 100, 000 km. The binary orbits have eccentricities from e = 0to ∼1, with the majority having e≥ 0.6. The binary orbit inclinations with respect to the initial angular momentum of the swarm range from i = 0to ∼90°, with most cases having i< 50°. 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 ∼ 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.