The wide-binary origin of (2014) MU69-like Kuiper belt contact binaries

Abstract

Following its flyby and first imaging the Pluto-Charon binary, the New Horizons spacecraft visited the Kuiper-Belt-Object (KBO) (486958) 2014 MU 69 (Arrokoth). Imaging showed MU 69 to be a contact-binary, made of two individual lobes connected by a narrow neck, rotating at low spin period (15.92 h), and having high obliquity (∼ 98 • ) 1 , similar to other KBO contact-binaries inferred through photometric observations 2 . The origin of such peculiar configurations is puzzling, and all scenarios suggested for the origins of contact-binaries [3][4][5] fail to reproduce such properties and their likely high frequency. Here we show that semisecular perturbations 6, 7 operating only on ultra-wide (∼ 0.1−0.4 Hill-radius 8 ) KBO-binaries can robustly lead to gentle, slow-speed binary mergers at arbitrarily high obliquities, but low rotational velocities, that can reproduce MU 69 's (and similar oblique contact binaries) characteristics. Using N-body simulations, we find that ∼ 15% of all ultra-wide binaries with cosine-uniform inclination distribution 5, 9 are likely to merge through this process. Moreover, we find that such mergers are sufficiently gentle as to only slightly deform the KBO shape, and can produce the measured rotation speed of MU 69 . The semi-secular contact-binary for-1

Investigating gravitational collapse of a pebble cloud to form transneptunian binaries

“…Regardless of the way it formed, a low-mass proto-Arrokoth binary system would have to subsequently evolve into a contact binary. As discussed by Stern et al (2019), McKinnon et al (2020 and Grishin et al (2020), the binary system would have had to lose angular momentum in order for the components to come together. Proposed mechanisms include gas drag, the Kozai-Lidov mechanism, tidal evolution, and collisions.…”

“…The collisional origin of Arrokoth was then investigated further by McKinnon et al (2020), who showed that a low-velocity impact can indeed preserve the bi-lobate structure. Furthermore, Grishin et al (2020) demonstrated that dynamical evolution of an initially wide binary can lead to an Arrokoth forming gentle merger. As pointed out by Nesvorný et al (2018b) for the case of the comet 67P, Simon et al (2017) provided evidence that streaming instability may scale down to produce less massive <100 km objects.…”

Surface Dynamics, Equilibrium Points and Individual Lobes of the Kuiper Belt Object (486958) Arrokoth

“…2, the large lobe has higher oblateness as well as elongation than the small lobe. The large lobe had an oblateness ∼ 1.72× greater than that observed by Grishin et al (2020). The small lobe had an approximately equal oblateness of 0.1 to that observed by Grishin et al (2020).…”

“…The large lobe had an oblateness ∼ 1.72× greater than that observed by Grishin et al (2020). The small lobe had an approximately equal oblateness of 0.1 to that observed by Grishin et al (2020). These authors used ellipsoidal models for the large and small lobes.…”

“…Each lobe has its own body-fixed co-ordinate frame for describing the locations of equilibrium points. We also assume that each individual lobe has a uniform rotation about its own largest moment of inertia (z-axis) along with their centroids with same spin period of 9. near the rotation period of 10 h used by Grishin et al (2020), for smoothed particles hydrodynamic simulations of the origin of Arrokoth-like Kuiper-belt contact binaries from wide binaries. All equilibrium points are computed for a constant density of ρ = 0.5 g cm −3 .…”