SIZE DISTRIBUTION FOR SUB-KM MAIN BELT
ASTEROIDS DEDUCED FROM SUBARU WIDE-FIELD
CAMERA OBSERVATIONS
YOSHIDA, F. 1,2, NAKAMURA, T. 1, FUSE, T. 3, KOMIYAMA, Y. 3, YAGI,
M. 1, MIYAZAKI, S. 1,OKAMURA, S. 4, OUCHI, M. 4, MIYAZAKI, M. 4, &
SUPRIME-CAM TEAM
1
National Astronomical Observatory of Japan,
. Kobe University, Japan,
3
. RCUH, USA,
4
University of Tokyo, Japan
2
Importance of sub-km (in diameter) objects among the main belt asteroids
(MBAs) in terms of their origin and collisional evolution has progressively
been recognized from the following viewpoints: 1) recent impact theories
suggest that the dividing line between monolith asteroids and rubble piles
will be near the sub-km region, 2) the majority of near-Earth asteroids which
are supposed to originate from the MBAs are smaller than 1km in size, and 3)
formation of rubble piles among the MBAs may affect their size distribution
in the sub-km range. Keeping the background in mind, we made preliminary
observations of the sub-km MBAs, whose primary purpose is to know the
slope of their cumulative size distribution. We observed a 27'x 27' sky of 41
deg off opposition near the ecliptic, with the wide-field mosaic camera
(Suprime-Cam) for the Subaru 8.2m telescope. We found out 27 MBAs in the
FOV, ranging from 14.5 to 19.5 mag in absolute magnitude (the faintest one
corresponds to about 600m). Although estimated H-mag for each detected
asteroid involves large errors due to both its unknown location in the belt and
mismatch of observed V-band with USNO-A2 catalog R-magnitude used in
data reduction, it is found that the slope of the cumulative size distribution for
the asteroids is fairly gentler than that for the asteroids with D > 1km
obtained from Palomar-Leiden (1970) and Spacewatch surveys (1998).
Moreover, the result seems to be consistent with an outcome from our recent
sub-km MBAs survey at opposition which detected approximately 1000
asteroids. Considering common occurrence of rubble piles among the MBAs
shown in impact simulations (e.g., Campo Bagatin et al., 2001), our gentle
slope in the size distribution for the sub-km MBAs could be interpreted as
substantial depletion of sub-km fragments in the belt caused by their
incorporation into rubble piles as building blocks.