A Collisional Family in the Classical Kuiper Belt

The dynamical evolution of Classical Kuiper Belt Objects (CKBOs) divides intotwo parts, according to the secular theory of test particle orbits. The firstpart is a forced oscillation driven by the planets, while the second part is afree oscillation whose amplitude is determined by the initial orbit of the testparticle. We extract the free orbital inclinations and free orbitaleccentricities from the osculating elements of 125 known CKBOs. The freeinclinations of 32 CKBOs strongly cluster about 2 degrees at orbital semi-majoraxes between 44 and 45 AU. We propose that these objects comprise a collisionalfamily, the first so identified in the Kuiper Belt. Members of this family areplausibly the fragments of an ancient parent body having a minimum diameter of\~800 km. This body was disrupted upon colliding with a comparably sizedobject, and generated ejecta having similar free inclinations. Our candidatefamily is dynamically akin to a sub-family of Koronis asteroids located atsemi-major axes less than 2.91 AU; both families exhibit a wider range in freeeccentricity than in free inclination, implying that the relative velocitybetween parent and projectile prior to impact lay mostly in the invariableplane of the solar system. We urge more discoveries of new CKBOs to test thereality of our candidate family and physical studies of candidate familymembers to probe the heretofore unseen interior of a massive, primitiveplanetesimal.Comment: final revised version, accepted to ApJ Letters, includes minor caveat regarding Koronis asteroid famil