Search for Fast-rotators among the Main-belt Asteroids

The existence of several fast-rotating objects with spin-periods less than T_r ( ~2 hr: a spin stability limit for strengthless rubble-pile asteroids) has recently been reported among near-Earth asteroids (NEAs); all of them are found to be smaller than about 200 m in diameter (Pravec and Harris 2000; Whiteley et al. 2002). Considering the now widely accepted view that NEAs originate dynamically from the main-belt asteroids (MBAs), it is natural for us to look for fast-rotators among the sub-km MBAs. However, because of their faintness, such an attempt had been unsuccessful with telescopes of a few meters in diameter, which were in common use a decade ago. With the wide-field mosaic CCD camera (Suprime-Cam) attached to the prime focus of the 8.2 m Subaru telescope atop Mauna Kea, Hawaii, we made lightcurve observations of very small MBAs on Oct. 20, 2001. In a FOV of 34' x 27' covered by ten chips, we detected 127 asteroids down to R ~25 and light variations for 92 asteroids were measured. Fourier periodogram analysis brought about 5 "certain"-class and 20 "likely"-class fast-rotators with spin periods less than T_r. Their sizes range from 0.5 to 2 km. Periodic nature of those data was confirmed by reanalysis of simulated lightcurves. The main results of this study are summarized as: 1) the MBA fast-rotators discovered in this observation are about one order of magnitude larger than those among NEAs, 2) lightcurve amplitudes for our MBA fast-rotators are small compared with those for ordinary asteroids, and this finding is consistent with the result on NEA fast-rotators (Pravec and Harris 2000), and 3) fast-rotators may occupy a non-negligible fraction ( ~25/92=27%) of MBAs in our observed size range.