Size and Spatial Distributionsof Sub-km Main-Belt Asteroids

Astronomy and Astrophysics – Astronomy

Scientific paper

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Asteroids, Main-Belt, Minor Planet, Size Distribution, Solar System: General, Spatial Distribution, Surveys

Scientific paper

This paper presents the result of the first systematic investigation of very small Main-Belt Asteroids (sub-km MBAs) using the Subaru Prime-Focus Camera (Suprime-Cam) with an 8K × 10K mosaic CCD array attached to the 8.2m Subaru Telescope atop Mauna Kea, Hawaii. We call this survey SMBAS (Sub-km Main-Belt Asteroid Survey). Observations were carried out on 2001 February 22 and 25 (HST) and a ˜ 3.0deg2 sky area near the opposition and near the ecliptic was searched. We detected 1111 moving objects down to R ˜ 26 mag (including very slow Trans-Neptunian Objects). In this survey, we could not determine the exact orbits of the moving objects, because of their short observational arc of only 2 hours. Instead, we statistically estimated the semi-major axis (a) and inclination (I) of each moving object from its apparent sky-motion vector, and then obtained the size and spatial distributions of sub-km MBAs. The main results of SMBAS are: (1) The sky number density of MBAs is found to be ˜ 290deg-2 down to R ˜ 24.4 mag (for MBAs) near the opposition and near the ecliptic. (2) The slope of the cumulative size distribution for sub-km MBAs ranging from 0.5km to 1km in diameter is fairly shallower (˜ 1.2) than that for large MBAs of more than ˜ 5km in diameter (˜ 1.8), which was obtained from past asteroid surveys. This means that the number of sub-km MBAs is much more depleted than a result extrapolated from the size distributions for large asteroids. (3) The depletion of sub-km MBAs is clearer in the outer main-belt than in the inner main-belt. (4) It seems that SMBAS asteroids distribute more widely in the I-direction in the outer zone (a = 2.8 - 3.1 AU) of the main-belt than known large asteroids do. We also discuss the possible causes for the characteristics of the distributions of SMBAS-observed small asteroids.

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