Presentation #206.02 in the session “Centaurs and Kuiper Belt Objects: Formation and Evolution”.
The apparent gentle merger of the two lobes of the cold classical Kuiper belt object (KBO) 486958 Arrokoth, as revealed by New Horizons (Stern et al., Science 364, eaaw9771, 2019; Spencer et al., Science 367, aay3999, 2020; Grundy et al., Science 367, aay3705, 2020), prompts consideration of the physical mechanism(s) that might have driven mergers of originally co-orbiting binaries (which are known to be common in the Kuiper belt today). In McKinnon et al. (Science 367, aay6620, 2020 [M20]) several mechanisms were examined: tides, collisions, Kozai-Lidov cycling, asymmetric radiation effects (YORP and BYORP), and gas drag. Here we examine and update the case for gas drag, both as it might have affected Arrokoth and more generally. The evolution of binaries in a gaseous protoplanetary disk was first considered in detail by Perets & Murray-Clay (Astrophys. J. 733, 56, 2011 [PMC]), who focused on the possibility of differential wind shear causing binaries to become unbound, and secondarily on the possibility that gas drag could cause binary inspiral and merger. The former is not an issue for the relatively massive lobes of Arrokoth, and we derive a new result for the latter. Though the protosolar nebular gas at Arrokoth’s distance is quite dilute and binary motions slow, it is the headwind due to pressure support of the nebula that determines the drag regime, irrespective of the binary’s orientation, and couples to the slower velocity of the co-orbiting binary. As the binary pinwheels in this nebular wind, each of its lobes will alternately feel accelerating and decelerating torques; time averaged the difference does not average to zero. M20 derived a stopping time (e-folding time of the binary’s angular momentum) for Arrokoth as low as 1-2 Myr, based on the classic Whipple-Weidenschilling drag formulae, recognizing that inspiral of an extended binary orbit could take longer than the nominal nebular lifetime of ~5 Myr. Using the standard drag law in PMC, and with a corrected estimate for the Reynolds number, lengthens these time scales further (and all such estimates are subject to the uncertainties in nebular and binary density). Nevertheless, we expect the importance of gas drag to increase for KBO binaries that are smaller, lower density, less inclined, and/or form closer to the Sun.