Colors and spin period distributions of sub-km main belt asteroids

The size dependency of space weathering on asteroid’s surface and collisional lifetimes suggest that small asteroids are younger than large asteroids. Therefore, the studies of smaller asteroid provide us new information about asteroid composition on fresh surface and their collisional evolution. We performed a color observation using 4 filters and lightcurve observation using 2 filters on different nights, using the 8.2m Subaru telescope/Suprime-Cam, for investigating the color and spin period distributions of sub-km main-belt asteroids (MBAs) that could not be seen before by middle class telescopes. In a lightcurve observation on Sep. 2, 2002, we kept taking images of a single sky field at near the opposition and near the ecliptic plane. Taking advantage of the wide field view of Suprime-Cam, this observation was planned to obtain lightcurves of 100 asteroids at the same time. Actually, we detected 112 MBAs and obtained their lightcurves by using a modified GAIA-GBOT PIPELINE. For the period analysis, we defined a criterion for judging whether an obtained rotational period is robust or not. Although Dermawan et al. (2011) have suggested that there are many fast rotators (FR) in MBAs, we noticed that many MBAs have long spin periods. Therefore, we could determine the rotation period of only 22 asteroids. We found one FR candidate (P=2.02 hr). We could measure the B-R color of 16 asteroids among the 22 MBAs. We divided them into S-like and C-like asteroids by the B-R color. The average rotational periods of C-like and S-like asteroids are 4.3 hr and 7.6 hr, respectively. C-like asteroids seem to rotate faster than S-like ones. We carried out a multi-color survey on Aug. 9 and 10, 2004 and then detected 154 MBAs. We classified them into several taxonomic types. Then we noticed that there are only very few Q-type candidates (non-weathered S-type) unlike the near Earth asteroid (NEAs) population, in which Q-type is a main component. This may indicate that most of Q-type NEAs did not originated from Q-type MBAs. They are probably objects subjected to resurfacing process (by peeling surface regolith, the outer layer of asteroid changes from S-type to Q-type) due to the tidal effect during their planetary encounters.