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W.J. Merline, P.M. Tamblyn, D. Nesvorny, D.D. Durda, C.R. Chapman (SWRI), C. Dumas (ESO), A.D. Storrs (Towson Univ.), B. Feldman (Haverford College), W.M. Owen (JPL), L.M. Close (U. Arizona), F. Menard (Obs. Grenoble)
We report the discovery of two binary systems among small, main-belt asteroids in the Koronis dynamical family. They were discovered using the Advanced Camera for Surveys of HST. Our program (SNAP-9747) was designed to determine binary frequency among two new exceptionally young families. Nesvorny et al. (2002 Nature 417, 720; 2003 ApJ 591, 486) used dynamical back-integrations to identify and precisely date the collisional breakup events of two families. Our goal was to determine if the families (Karin, S-type, age 5.8 +/- 0.2 MY and Veritas, C-type, age 8.3 +/- 0.5 MY) have a higher binary fraction than two control populations. Each control group was selected to have similar orbital elements to those of the Karin cluster, and therefore would see similar collisional environments. One of the groups is an ''old" family (few BY), Koronis (of which the Karin family itself if a sub-family, i.e. the Karin-family progenitor was created from the Koronis-family breakup). The other group is a collection of background asteroids that do not have an identified family association. Our expectation was that a very young family might have a significantly higher frequency of binaries than the background, because the satellites may not yet have been destroyed by subsequent impact or longer-term gravitational instabilities. In fact, one of the prime observables from such an event might be the propensity for satellites. To our surprise, we have found two binaries among the Koronis (old) population and none among the others, from our rather small sample (17 Karin, 18 Veritas, 9 Koronis, 9 background). We requested 180 targets and expected to observe more than 90 in our Cycle 12 program, although only 53 have been observed so far. The satellites were discovered around (22899) (1999 TO14) (Merline et al., IAUC 8232) and (17246) (2000 GL74) (Tamblyn et al., IAUC 8293), both of Vmag about 18. The primaries of both systems are S-type asteroids of approximate diameter 4.5 km. S/2003(22899)1 is about 1.5 km diameter (the size of Dactyl) and orbits at about 75 R(primary) (170 km), while S/2004(17246)1 is about 2 km diameter and orbits at about 100 R(primary) (230 km). We estimate the orbital periods to be about 56 days for S/2003(22899)1 and 90 days for S/2004(17246)1. We have an approved follow-on program to determine colors and orbital parameters using the same instrument (GO-10165, Cycle 13). These are the only other binaries known among the Koronis family, after Ida/Dactyl. Given the large number of initial binaries expected from large collisions in hydrocode simulations, such as those being done now by Durda et al. (2004 Icarus 170, 243), our results indicate that the size of the progenitor (large for Koronis, small for Karin) may be a more important factor than age in the number of satellites produced and retained. These are the smallest main-belt asteroids known to harbor companions. Study of a variety of binary systems is the only way to calibrate numerical models for binary production by collisions.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.