ALBEDO PROPERTIES OF MAIN BELT ASTEROIDS BASED ON THE ALL-SKY SURVEY OF THE INFRARED ASTRONOMICAL SATELLITE<i>AKARI</i>

Abstract

We present an analysis of the albedo properties of main belt asteroids\ndetected by the All-Sky Survey of the infrared satellite AKARI. The\ncharacteristics of 5120 asteroids detected by the survey, including their sizes\nand albedos, were cataloged in the Asteroid Catalog Using AKARI (AcuA). Size\nand albedo measurements were based on the Standard Thermal Model, using inputs\nof infrared fluxes and absolute magnitudes. Main belt asteroids, which account\nfor 4722 of the 5120 AcuA asteroids, have semimajor axes of 2.06 to 3.27 AU.\nAcuA provides a complete data set of all main belt asteroids brighter than the\nabsolute magnitude of H &lt; 10.3, which corresponds to the diameter of d &gt; 20 km.\nWe confirmed that the albedo distribution of the main belt asteroids is\nstrongly bimodal as was already known from the past observations, and that the\nbimodal distribution occurs not only in the total population, but also within\ninner, middle, and outer regions of the main belt. We found that the small\nasteroids have much more variety in albedo than the large asteroids. In spite\nof the albedo transition process like space weathering, the heliocentric\ndistribution of the mean albedo of asteroids in each taxonomic type is nearly\nflat. The mean albedo of the total, on the other hand, gradually decreases with\nan increase in semimajor axis. This can be explained by the compositional ratio\nof taxonomic types; that is, the proportion of dark asteroids such as C- and\nD-types increases, while that of bright asteroids such as S-type decreases,\nwith increasing heliocentric distance. The heliocentric distributions of\nX-subclasses: E-, M-, and P-type, which can be divided based on albedo values,\nare also examined. P-type, which is the major component in X-types, are\ndistributed throughout the main belt regions, and the abundance of P-type\nincreases beyond 3 AU. This distribution is similar to that of C- or D-types.\n