First Subaru observations of sub-km main-belt asteroids

We report on the first observations of sub-km main-belt asteroids (MBAs) ob tained with the prime-focus mosaic CCD camera for the Subaru telescope. We detected 27 moving objects in a single image of the sky (field of view: 27 x 27). whose location was 41 degrees off opposition. From their positions a nd projected motions on the sky, all of the detected objects were found to be new and consistent with the characteristic of MBAs. The V-magnitudes of the discovered asteroids range approximately from 19 to 24. Under some simp le., but reasonable, assumptions, we estimated the cumulative size distribu tion for the asteroids. This is the first statistics for sub-km MBAs (the m inimum diameter corresponds to about 0.6 km). It is found that the slope of the cumulative size distribution for the observed asteroids is 1.0 with an error of about 0.3. We note that our slope is fairly smaller than that obt ained in the past observational surveys for asteroids larger than a few kil ometers (similar to 1.75).