Formation of Kuiper Belt Binaries by Gravitational Collapse

Abstract

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 QW 322 and multiples such as (47171) 1999 TC 36 . 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 = 0 to ∼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 = 0 to ∼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.

Rotational properties of the binary and non-binary populations in the trans-Neptunian belt

“…Various models have been proposed to explain the formation of binary and multiple systems, such as the L 3 mechanism by Goldreich et al (2002), the L 2 s mechanism by Goldreich et al (2002), the chaos-assisted capture by Astakhov et al (2005), the collisional model by Durda et al (2004), the hybrid model by Weidenschilling (2002), the gravitational collapse by Nesvorný et al (2010), and the rotational fission model by Ortiz et al (2012b). A complete review of some of these mechanisms can be found in Noll et al (2008).…”

“…Sheppard et al (2012) previously discussed in detail all possible (or impossible) formation models for this system. They considered two plausible scenarii: the L 3 mechanism based on gravitational capture proposed by Goldreich et al (2002), and the gravitational collapse mechanism studied by Nesvorný et al (2010).…”

Formation of pebble-pile planetesimals

“…An advantage with gravitational instability is that, if the internal angular momentum of a gravitationally unstable pebble cloud is large, it naturally forms a binary planetesimal with mass ratio of ∼1 (Nesvorný et al 2010). The components form from the same material, so they will also have the same chemical composition and colour, in agreement with what is observed for binary Kuiper belt objects (Benecchi et al 2009).…”

“…The Kuiper belt as a whole has a wide colour distribution, so this suggests that the components in binary Kuiper belt objects formed together; a similar co-formation explanation is not possible in models of binary formation through three-body encounters (Goldreich et al 2002) in the hierarchical coagulation formation mechanism. Nesvorný et al (2010) modelled the particle cloud with an N-body method with perfect sticking. The goal of this paper is to use more realistic collision outcomes based on laboratory experiments to model the collapse.…”

Facula, Faculae

“…The outcomes of these collisions determine the sizes of the planetesimals which emerge from the contracting clouds. Nesvorny et al (2010) investigated the outcome of collapsing pebble clouds and found that an initial mean rotation of the cloud (which could be inherited from the overall rotation of the disk) leads typically to the formation of a binary planetesimal surrounded by particles clumps which can be interpreted as a swarm of smaller planetesimals. This model agrees with the observation that the classical cold component of the Kuiper belt -the most pristine planetesimals in the Solar System which have undergone very little collisional and dynamical evolution -have a very high binary fraction.…”