[Previous] | [Session 2] | [Next]
W.J. Merline (SwRI), L.M. Close, N. Siegler (U. Arizona), C. Dumas (JPL), C.R. Chapman (SwRI), F. Rigaut (Gemini Observatory), F. Menard (Obs. Grenoble), W.M. Owen (JPL), D.C. Slater, D.D. Durda (SwRI)
We report the discovery of a small, distant companion to (3749) Balam from J, H, and K' imaging with the Hokupa'a adaptive optics system on the 8.1-m Gemini North Telescope on Mauna Kea. On 2002 Feb 8.4 UT we detected a satellite separated by 0.47" (projected separation 350 km) and differing in brightness by 3.3 mag in H (Merline et al., IAUC 7827). The primary is estimated to be 7 km in diameter, and thus the secondary would be expected to be about 1.5 km diameter. At time of discovery, the system was 2.01 AU from the Sun and 1.03 AU from Earth. The primary is about 20 times smaller than than the next-largest primary in a binary system observed from Earth. At Vmag=15.5, this is certainly the faintest main-belt binary yet detected from Earth. No motion could be detected over a 3.1-hr baseline, consistent with the large orbit. Insufficient additional observations have been obtained to determine the orbital size or period. Assuming a circular orbit, a lower limit to the orbit size would be the projected separation of 350 km. Further assuming a density of 2.6 g/cc for the primary, we estimate a period of about 85 days. With a separation of over 100 primary radii and being in the inner main-belt, this object is the most loosely-bound binary known, even more so than the TNO binaries. All other main-belt binaries orbit at 4-12 radii, suggesting that Balam formed by a very different process. It is probably the first example of an object formed by 'disruptive capture', first discussed by Hartmann (1979, in Asteroids, p. 466) and popularized by the work of Durda (1996, Icarus 120, 212) and Doressoundiram et al. (1997, Planet. Space. Sci. 45, 757). Described by Weidenschilling et al. (1989, in Asteroids II, p. 643) and Merline et al. (2002, in Asteroids III, in press), it occurs when two fragments from a catastrophic disruption leave the impact scene as a co-orbiting bound pair. Most other known main-belt binaries were probably formed from reaccumulated ejecta from a large cratering event. Balam is a member of the Flora family, indicating that it participated in a catastrophic breakup. Like Flora, it is probably an S-type asteroid, and thus joins Ida/Dactyl as the only S-types among the many C-like main-belt binaries. This work is supported by NSF, NASA, and Gemini Observatory.
If the author provided an email address or URL for general inquiries,
it is as follows:
Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.