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S.I. Ipatov (George Mason University)
We consider [1] that many trans-Neptunian objects (TNOs) with diameter d>100 km (and even Pluto and Charon) moving now in not very eccentric (e<0.3) orbits could be formed directly by the compression of large rarefied dust condensations (with a>30 AU), but not by the accretion of smaller solid planetesimals. Probably, some planetesimals with d~100-1000 km in the feeding zone of the giant planets and with d~100 km in the terrestrial planets' zone and some large main-belt asteroids could also be formed directly by such compression. Some smaller objects (TNOs, planetesimals, asteroids) could be debris of larger objects, and other such objects could be formed directly by compression of condensations. As in the case of accumulation of planetesimals, there could be a ``run-away" accretion of condensations and there was a distribution in masses of final condensations, which compressed into planetesimals. Five years before the first TNO was observed, based on our runs of formation of the giant planets we supposed [2] that there were two groups of TNOs and, besides TNOs formed beyond 30 AU and moving in low eccentric orbits, there were former planetesimals from the zone of the giant planets in highly eccentric orbits beyond Neptune. It is considered that TNO binaries can be produced due to the gravitational interactions or collisions of future binaries with an object (or objects) that entered their Hill sphere. To our opinion, binary TNOs (including Pluto-Charon) were probably formed at that time when orbits of TNOs were almost circular. For such orbits, two TNOs entering inside their Hill sphere could move there for a long time (e.g., greater than half orbital period [2]). We suppose that a considerable portion of TNO binaries could be formed at the stage of compressing of condensations. At this stage, the diameters of condensations, and so probabilities of their mutual collisions and probabilities of formation of binaries were much greater than those for solid TNOs. The stage of condensations was longer for TNOs than that for asteroids, and therefore binary asteroids (which could be mainly formed after the formation of solid objects) are less frequent and more differ in mass than binary TNOs. Besides, at the initial stage of solar system formation, eccentricities of asteroids could be mainly greater (due to the influence of the forming Jupiter and planetesimals from its feeding zone) than those of TNOs. This work was supported by NASA grant NAG5-10776, INTAS (00-240) and RFBR (01-02-17540). [1] Ipatov S.I., LPSC, 2001, #1165; [2] Ipatov S.I., Earth, Moon, and Planets, 1987, v. 39, 101-128.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.