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W. F. Bottke (SwRI), D. Vokrouhlicky, M. Broz (Charles U.), D. Nesvorny (SwRI), A. Morbidelli (Obs. Nice)
Asteroid families are remnants of large-scale catastrophic collisions. They are identified by their orbital elements, which tend to be clustered at similar values. Studies of asteroid families suggest that most fragments were collisionally ejected at high velocities, with the smallest fragments traveling the furthest from the cluster-center. Curiously, these high velocities have yet to be reproduced in numerical hydrocodes, which have been validiated against laboratory experiments and nuclear explosions. Whenever high velocities are obtained in hydrocode models, the fragment distribution is dominated by small-and thus unobservable-fragments.
We suggest an alternative scenario to explain this discrepancy. Collisional models suggest that many promient asteroid families are hundreds of Myrs to Gyrs old. From the time of their formation, asteroid families have had their configurations modified by collisions, chaotic diffusion, and, in particular, the Yarkovsky effect. Yarkovsky thermal forces cause D < 20 km bodies to slowly drift inward/outward in semimajor axis. The drift rate is size dependent, such that km-sized family members tend to move faster than multi-km bodies. We believe this effect spreads asteroid families over long time scales while also giving them the appearance that they were launched using a size-dependant velocity distribution.
To study this problem, we have numerically integrated test members of the Koronis family using the code SWIFT-RMVS3, which was modified to include the Yarkovsky effect. Our test asteroids were given a much more compact (a,e,i) distribution than observed among the Koronis family today and were then tracked for up to 600 Myr. Our results suggest Yarkovsky forces drive many family members through a chaotic sea where resonant jumping/trapping events produce small but noticable changes in proper e, i. We believe our simulation reproduces the unique distribution and span of the Koronis family while also showing that some Koronis family members could be escaping out of powerful resonances today.