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

Abstract

We have investigated effects of early stellar encounters on a protoplanetary disk (planetesimal disk) and found that they can explain the high eccentricities and inclinations observed in the outer part ($>42$AU) of the Edgeworth-Kuiper Belt (EKB). The proto-sun is considered as a member of a stellar aggregation that undergoes dissolution on a timescale $\sim 10^8$yrs, such that a planetesimal disk experiences a flyby encounter at pericenter distance ($q$) on the order of 100AU. We simulated dynamical evolution of a planetesimal (test particle) disk perturbed by a passing star. We show that the stellar encounter pumps the velocity dispersion in the disk in the outer parts ($> 0.25q$). Planet formation is forestalled in that region. We also find that a stellar encounter with pericenter distance $q \sim 100-200$AU could have pumped up the velocity dispersion of EKB objects outside 42AU to the observed magnitude while preserving that inside Neptune's 3:2 mean-motion resonance (located at 39.5AU), which allows for the efficient capture of objects by the sweeping of the 3:2 resonance during orbital migration by proto-Neptune.