Evidence for early stellar encounters in the orbital distribution of Edgeworth-Kuiper Belt objects

We show that early stellar encounters can explain the high eccentricities a nd inclinations observed in the outer part (>42 AU) of the Edgeworth-Kuiper Belt (EKB). We consider the proto-Sun as a member of a stellar aggregation that undergoes dissolution on a timescale of similar to 10(8) yr, such tha t the solar nebula experiences a flyby encounter at pericenter distance (cl ) on the order of 100 AU. Using numerical simulations we show that a stella r encounter pumps up the velocity dispersion in the young solar nebula in t he outer parts. In the case of a nearly parabolic encounter with a solar ma ss companion, the velocity dispersion at a greater than or equal to 0.25q i s pumped up to such an extent that collisions between planetesimals would b e expected to become highly disruptive, halting further growth of planetesi mals. This has the consequence that planet formation is forestalled in that region. We also find that a stellar encounter with pericenter distance q s imilar to 100-200 AU could have pumped up the velocity dispersion of EKB ob jects outside 42 AU to the observed magnitude while preserving the velocity dispersion magnitude inside Neptune's 3:2 mean motion resonance (located a t 39.5 AU). This allows for the efficient capture of objects by the resonan ce during a phase of orbital migration by proto-Neptune, which we also test with simulations. We point out that such a stellar encounter generally aff ects the dynamical and material structure of a protoplanetary disk, and the planetesimal distribution can remain imprinted with this signature over mu ch of the main-sequence lifetime of the star. In particular, our results su pport the notion that an analogous process has operated in some recently ob served extrasolar dust disks.