Formation of the extreme Kuiper-belt binary 2001 QW322 through adiabatic switching of orbital elements

Binaries in the Kuiper-belt are unlike all other known binaries in the Solar System. Both their physical and orbital properties are highly unusual and, because these objects are thought to be relics dating back to the earliest days of the Solar System, understanding how they formed may provide valuable insight into the conditions which then prevailed. A number of different mechanisms for the formation of Kuiper-belt binaries (KBBs) have been proposed including; two-body collisions inside the Hill sphere of a larger body; strong dynamical friction; exchange reactions; and chaos assisted capture. So far, no clear consensus has emerged as towhich of thesemechanisms (if any) can best explain the observed population of KBBs. Indeed, the recent characterization of the mutual orbit of the symmetric (i.e., roughly equalmass)KBB2001QW322 has only served to complicate the picture because its orbit does not seem readily explicable by any of the available models. The binary 2001 QW322 stands out even among the already unusual population of KBBs for the following reasons: its mutual orbit is extremely large (≈105 km or about 30% of the Hill sphere radius), retrograde, it is inclined ≈120◦ from the ecliptic and has very low eccentricity, i.e., e ≤ 0.4 (and possibly e ≤ 0.05). Here we propose a hybrid formation mechanism for this object which combines aspects of several of the mechanisms already proposed. Initially two objects are temporarily trapped in a long-living chaotic orbit that lies close to a retrograde periodic orbit in the three-dimensional Hill problem. This is followed by capture through gravitational scattering with a small intruder object. Finally, weak dynamical friction gradually switches the original orbit “adiabatically” into a large, almost circular, retrograde orbit similar to that actually observed. A. Gamboa Suárez · D. Farrelly (B) Utah State University, 0300 Old Main Hill, Logan, UT 84322, USA e-mail: [email protected]; [email protected] A. Gamboa Suárez e-mail: [email protected] D. Hestroffer IMCCE/Paris Observatory, Paris, France e-mail: [email protected]