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W.J. Merline (SwRI), P.M. Tamblyn (Binary Astronomy and SwRI), D. Nesvorny, D.D. Durda, C.R. Chapman (SwRI), C. Dumas (ESO (Chile)), W.M. Owen (JPL), A.D. Storrs (Towson Univ.), L.M. Close (U. Arizona), F. Menard (Obs. Grenoble (France))
Our ground-based adaptive optics observations of many larger Koronis members show no binaries, while our HST survey of smaller Koronis members (say smaller than 10 km) shows a surprising 20% binary fraction. Admittedly, this is from small-number statistics, but we nonetheless calculate a 99% confidence that the binary fraction is different from the 2% we observe among the larger (over 20km) main belt asteroids as a whole. In addition, we estimate that among the two young families (Karin and Veritas) that we surveyed for binaries in our HST Cy 13 program, the binary fraction appears to be less than 5%. These young families both have significantly smaller progenitors than the Koronis family. We have speculated that progenitor size may be a more important factor than age in determination of binary frequency. But here we suggest an alternative idea, that the binary fraction may be more related to what part of the family's size distribution is sampled. Our HST program targeted objects of the same physical sizes, but was clearly sampling further down the size distribution (to smaller sizes, relative to the largest remnant) in the Koronis sample than was the case for Karin and Veritas, which we sampled mostly at the larger sizes, relatively. Our SPH collision models are estimating the typical size-frequency distributions to be expected from catastrophic and non-catastrophic impact events. But they are also appear to be showing that the largest fragments from a collision are less likely to form binaries (as co-orbiting ejecta pairs) than are the smaller fragments. Thus, it might be expected that we would have found fewer binaries among Karin and Veritas than among the Koronis sample. In fact, models of the Karin breakup show binary formation to be unlikely in the size range measured. It some might be tempted to tie the small end of the main-belt binary population to the binaries seen among the NEAs (also small and also showing about 20% fraction), given the 20% fraction among small Koronis objects, our own ground-based AO observations that smaller main-belt asteroids are showing a binary fraction larger than 2%, and the high binary fraction being seen among small main-belt asteroids being studied by lightcurve analysis. But we believe this may be simply coincidental, given the vastly different dynamical and collisional regimes and lifetimes of the two populations.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.