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W.J. Merline (SwRI), L.M. Close (U. Arizona), C. Dumas (JPL), C.R. Chapman (SwRI), F. Menard (Obs. Grenoble, France), P.M. Tamblyn (Binary Astronomy and SwRI), D.D. Durda (SwRI)
We report on the discoveries of two new main-belt binary asteroids over the past year. On 2002 Sep 28, asteroid (121) Hermione, a C-type Cybele of diameter 209 km, was observed to have a satellite of approximate diameter 13 km with a projected separation of 630 km. These observations were made with the Keck II telescope and the NIRC2 camera with adaptive optics. Although insufficient data exist at this point to determine an orbit, the characteristics place this object into the same class as most other main-belt binaries: high mass ratio (several hundred to a few tens of thousands) and close orbits (semi-major axis about 10 primary radii). These objects fit well the predicted characteristics of binaries formed by reaccretion, in orbit, of ejecta from large cratering events, as suggested by Weidenschilling et al. (1989, Asteroids II, p. 643). Numerical simulations by Durda et al. (2003, LPSC 34, 1943) are now showing efficient production of such binaries, which they are calling SMATS (SMAshed Target Satellite). We also discovered that (1509) Esclangona, a small (12 km) inner-main-belt asteroid, was binary on 2003 Feb 13. Again, there is insufficient data to determine an orbit, but the companion appears to orbit at least 23 primary radii from Esclangona. Such a wide orbit could not have resulted from tidal evolution from a close binary formed by the SMATS process (Weidenschilling et al. 1989). Therefore this objects joins (3749) Balam as a member of a second class of objects that are characterized by wide separations (23 and 100 primary radii in the two examples), and less extreme mass ratios (27 and 95 here) than for the SMAT binaries. These binaries were suggested by Hartmann (1979, Asteroids, p. 466) and further described by Weidenschilling et al. and are comprised of two ejecta fragments from a catastrophic disruption that leave the scene as a co-orbiting pair. The models of Durda et al. are also showing that these binaries, which they term EEBs (Escaping Ejecta Binaries), can be efficiently produced.
The author(s) of this abstract have provided an email address for comments about the abstract: merline@boulder.swri.edu
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.