Sphericity Preference in Shapes of Sub-km-Sized Fast-Rotating Main-Belt Asteroids

Here, we report on results of statistical analyses on spin periods and shapes of main-belt asteroids (MBAs) smaller than similar to 1 km in diameter, which were derived from lightcurve observations of a single night with the 8.2 m Subaru telescope. It is shown that, among our detected 68 MBAs giving reliable lightcurve periods, the fraction of fast-rotating asteroids (with spin periods of < similar to 2.3 hr, candidates of monolithic bodies) is about 49%. In particular, statistical tests reveal with a high confidence level that, in terms of the shape (deduced from the light variation amplitude), the fast rotators belong to a population quite different from that for non fast-rotators; in other words, most of the fast-rotators show a strong trend that they are more spherical in shape than the non fast-rotator group. Consideration of several orbital and rotational evolution timescales for small asteroids in the main asteroid belt seems to indicate that the above-mentioned trend is not by coincidence, but primordial. Therefore, referring to the shape distributions of impact fragments produced in laboratory experiments, our discovered sphericity preference of small fast-rotating asteroids probably requires some spin deceleration mechanisms, which selectively worked on all elongated objects during their impact formation and/or subsequent evolution. We also note that our fast rotators are much more spherical compared to known small MBAs and near-Earth objects.