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Title:
Subaru Main Belt Asteroid Survey (SMBAS)—Size and color distributions of small main-belt asteroids
Authors:
Yoshida, F.; Nakamura, T.
Affiliation:
AA(Subaru Telescope, National Astronomical Observatory of Japan, Osawa, 2-21-1, Mitaka, Tokyo 181-8588, Japan), AB(The University of the Air, 2-11, Wakaba, Mihama, Chiba 261-8586, Japan)
Publication:
Planetary and Space Science, Volume 55, Issue 9, p. 1113-1125. (P&SS Homepage)
Publication Date:
06/2007
Origin:
ELSEVIER
Abstract Copyright:
Elsevier Ltd
DOI:
10.1016/j.pss.2006.11.016
Bibliographic Code:
2007P&SS...55.1113Y

Abstract

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˜4.0deg 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˜24.2 mag. It corresponds to H˜20 mag at the outer edge of main belt. Thus, assuming their mean albedos, down to 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, D: diameter) for sub-km MBAs ranging from 0.6 to 1 km in diameter is 1.29±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>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 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 (carbonaceous). 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 S- and 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.
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