Subaru Lightcurve Observations of Sub-km-Sized Main-Belt Asteroids

Abstract The currently known spin period vs size distribution of asteroids includes a very small number of sub-km-sized main-belt asteroids (MBAs). In order to get a better understanding of such objects, we conducted lightcurve observations using the wide-field camera (Suprime-Cam) of the 8.2 m Subaru telescope in 2001 October. We surveyed a sky field of 34$'$$\times$ 27$'$ near opposition and the ecliptic, covering a time-span of 8.3 hr for a single night, and detected 127 asteroids down to a limiting magnitude of $R$$\approx$ 24.6 mag (at a level of 90% fractional detection). Since the majority of our lightcurves consisted of data points of only a few tens, we took an approach with an emphasis on searching for a dominant period of a lightcurve using two independent periodogram codes, to ascertain the mutual consistency of the obtained results. In an attempt to estimate their period errors, we made extensive simulations using artificial lightcurve data, and found that most errors do not exceed $\sim$ 0.1 hr; this level of errors is sufficiently small for the purposes of this survey observation. Out of 68 main-belt asteroids showing reliable spin periods, 33 are found to be fast-rotator MBAs, whose spin periods range from 0.5 to 2.2 hr (hereafter asteroids with periods $\lt$ 2.2 hr are abbreviated as FRAs). This result is the first observational confirmation on the existence of FRAs among sub-km MBAs; those detected small MBAs fill up the region that used to be almost void of asteroids in the spin period vs size distribution. Regarding the shapes of those objects, which were inferred from their lightcurve amplitudes, we found a tendency that FRAs are more spherical than non-FRAs. Cumulative size distributions of the FRAs and non FRAs are also compared, but revealing no clear difference.