Subaru Main Belt Asteroid Survey (SMBAS)—Size and color distributions of small main-belt asteroids

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∼4.0deg2 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∼25mag. In SMBAS-II, we could not determine the exact orbits of the objects, because of the short observational arc of only ∼40min. 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∼24.2R∼24.2 mag. It corresponds to H∼H∼20 mag at the outer edge of main belt. Thus, assuming their mean albedos, down to D∼D∼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)∝D-bN(>D)∝D-b, D: diameter) for sub-km MBAs ranging from 0.6 to 1 km in diameter is 1.29±0.021.29±0.02. Our b   value (1.29) is much shallower than those (∼∼ 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>5D>5 km) and almost consistent with that (1.3) of SDSS [Ivezić, Ž., 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., Brinkmann, 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 kmD>0.6 km was 1:1 in the inner belt (2.0 AU