FORMATION OF KUIPER BELT BINARIES BY GRAVITATIONAL COLLAPSE

Abstract

A large fraction of 100-km-class low-inclination objects in the classical\nKuiper Belt (KB) are binaries with comparable mass and wide separation of\ncomponents. A favored model for their formation was capture during the\ncoagulation growth of bodies in the early KB. Instead, recent studies suggested\nthat large objects can rapidly form in the protoplanetary disks when swarms of\nlocally concentrated solids collapse under their own gravity. Here we examine\nthe possibility that KB binaries formed during gravitational collapse when the\nexcess of angular momentum prevented the agglomeration of available mass into a\nsolitary object. We find that this new mechanism provides a robust path toward\nthe formation of KB binaries with observed properties, and can explain wide\nsystems such as 2001 QW322 and multiples such as (47171) 1999 TC36. Notably,\nthe gravitational collapse is capable of producing 100% binary fraction for a\nwide range of the swarm's initial angular momentum values. The binary\ncomponents have similar masses (80% have the secondary-over-primary radius\nratio >0.7) and their separation ranges from ~1,000 to ~100,000 km. The binary\norbits have eccentricities from e=0 to ~1, with the majority having e<0.6. The\nbinary orbit inclinations with respect to the initial angular momentum of the\nswarm range from i=0 to ~90 deg, with most cases having i<50 deg. Our binary\nformation mechanism implies that the primary and secondary components in each\nbinary pair should have identical bulk composition, which is consistent with\nthe current photometric data. We discuss the applicability of our results to\nthe Pluto-Charon, Orcus-Vanth, (617) Patroclus-Menoetius and (90) Antiope\nbinary systems.\n