Since February, 2001, we have been conducting a series of survey observations to investigate the physical property of very small Main Belt Asteroids (sub-km MBAs) using the Subaru prime-focus camera (Suprime-Cam) attached to the 8.2 m Subaru telescope. We call our surveys "SMBAS: Subaru Main-Belt Asteroids Survey". This paper presents the results of the second SMBAS (SMBAS-II) which was performed in October 2001. In SMBAS-II, a similar to 4.0 deg(2) sky area near the opposition and near the ecliptic was surveyed with the R- and B-bands. We detected 1838 moving objects up to R similar to 25 mag. In SMBAS-II, we could not determine the exact orbits of the objects, because of the short observational arc of only similar to 40 min. Instead, we statistically estimated the semi-major axis (a) of each moving object from its apparent sky-motion vector assuming its circular orbit and then, we used the a's to select MBAs and to estimate their absolute magnitudes (H). The limiting magnitude of SMBAS-II for MBAs was R similar to 24.2 mag. It corresponds to H similar to 20 mag at the outer edge of main belt. Thus, assuming their mean albedos, down to D similar to 0.3 km of S-type asteroids and 0.6 km of C-type asteroids were detected in SMBAS-II. We found that the slopes (b) of the cumulative size distribution (CSD) (i.e. N(> D) alpha D-b, D: diameter) for sub-km MBAs ranging from 0.6 to 1 m in diameter is 1.29 +/- 0.02. Our b value (1.29) is much shallower than those (similar to 1.8) of the Palomer Leiden Survey (PLS) [van Houten, C.J., van Houten, G.I., Herget, P., Gehrels, T., 1970. The Palomar-Leiden survey of faint minor planets. Astr. Astrophys. Suppl. 2, 339-448] and Spacewatch surveys [Jedicke, R., Metcalfe, T.S., 1998. The orbital and absolute magnitude distributions of main belt asteroids. Icarus 131, 245-260.] for larger asteroids (D > 5 km) and almost consistent with that (1.3) of SDSS [Ivezic Z., Tabachnik, S., Rafikov, R., Lupton, R.H., Quinn, T., Hammergren, M., Eyer, L., Chu, J., Armstrong, J.C., Fan, X., Finlator, K., Geballe, T.R., Gunn, J.E., Hennessy, G.S., Knapp, G.R., Leggett, S.K., Munn, J.A., Pier, J.R., Rockosi, C.M., Schneider, D.P., Strauss, M.A., Yanny, B., Brinkmarm, J., Csabai, I., Hindsley, R.B., Kent, S., Lamb, D.Q., Margon, B., McKay, T.A., Smith, J.A., Waddel, P., York, D.G. (for the SDSS Collaboration), 2001. Solar system objects observed in the Sloan Digital Sky Survey commissioning data. Astron. J. 122, 2749-2784] for MBAs with 0.4 km < D < 5 km. This means that the number of sub-km MBAs is much more depleted than the result extrapolated from the size distribution for larger asteroids. This shallow slope of sub-km MBAs has been obtained from our first SMBAS (SMBAS-I) (b = 1.19 +/- 0.02) which was carried out in February 2001 [Yoshida, F., Nakamura, T., Watanabe, J., Kinoshita, D., Yamamoto, N., Fuse, T., 2003. Size and spatial distributions of sub-km main-belt asteroids. Publ. Astron. Soc. Jpn. 55, 701-715]. We examined taxonomic distribution of sub-km MBAs. Although we could not exactly distinguish S-type and C-type asteroids only using the R and B-magnitude, we divided SMBAS-II MBAs into two groups: S-like asteroids (rocky) and C-like asteroids (carbonaccous). The heliocentric distribution of S-like MBAs was almost flat throughout the entire main belt, while that of C-like MBAs shows an asymmetric distribution, namely, the number of C-like asteroids increases with the heliocentric distance. The number ratio of Sand C-like MBAs with D > 0.6 km was 1 : 1 in the inner belt (2.0 AU < a < 2.6 AU), 1 : 2.3 in the middle belt (2.6 AU < a < 3.0 AU), and 1 : 4 in the outer belt (3.0 AU < a < 3.5 AU), respectively. For the entire main belt, the ratio was 1 : 2.3. (c) 2006 Elsevier Ltd. All rights reserved.
