[U-06_28PM2] New Progress toward the Understanding of Small Solar System Bodies

★ [U06-18] Albedo properties of main belt asteroids based on the infrared all-sky surveyors

*Fumihiko USUI¹ (1.University of Tokyo)

Keywords:asteroids, main belt, infrared surveys, size and albedo, taxonomic classifications

Presently, the number of asteroids is known to be more than 620,000, and more than 90% of asteroids are classified as the main-belt asteroids (MBAs). The spatial distribution of compositions among MBAs is of particular interest, because the main belt is the largest reservoir of asteroids in the solar system. Asteroids are thought to be the remnants of planetesimals formed in the early solar system, and have a clue to study the formation and evolution of asteroids, origin of meteoroids and the near-Earth asteroids, as well as the formation of the solar system. Size and albedo are one of the most basic physical quantities of asteroid. Knowledge of size and albedo is essential in many fields of asteroid research, such as chemical composition and mineralogy, the size-frequency distribution of dynamical families and populations of asteroids, and the relationship between asteroids in the outer solar system and comets.Several techniques have been developed to determine the size of asteroids. One of the most effective methods for measuring asteroidal size and albedo indirectly is through the use of radiometry, where a combination of the thermal infrared flux and the absolute magnitude as the reflected sunlight. Using radiometric measurements, a large number of objects can be observed in a short period of time, providing coherent data for large populations of asteroids within the asteroid belt. Infrared observations can be made still better under ideal circumstances, from space. The first space-borne infrared telescope is the Infrared Astronomical Satellite (IRAS; Neugebauer et al. 1984), launched in 1983 and performed a survey of the entire sky. To date, there are two other infrared astronomical satellites dedicated to all-sky survey: the Japanese infrared satellite AKARI (Murakami et al. 2007), and the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010). Based on the all-sky survey data obtained by IRAS, AKARI, and WISE, the largest asteroid catalogs containing size and albedo data were constructed (e.g., Tedesco et al. 2002; Usui et al. 2011; Mainzer et al. 2011). The total number of asteroids detected with size and albedo information with these three surveyors is 138,285, which is 22% of currently known asteroids with orbits.In addition, several outstanding works have provided the taxonomic classification of asteroids (e.g., Tholen 1989; Bus & Binzel 2002; Lazzaro et al. 2004; Carvano et al. 2010), based on ground-based spectroscopic observations within optical and near-infrared wavelengths. Along with these taxonomic classifications, size and albedo data also contribute to our understanding of asteroid compositions. In general, the albedo of C-types is considered as low and that of S-types is high (e.g., Zellner & Gradie 1976). The relationship between taxonomic types and albedo is, however, complex and type determinations cannot be made on the basis of albedo values alone. Recently albedos of C- and S-type asteroids are found to vary widely, especially for sizes smaller than several tens km (Usui et al. 2013). Furthermore, in spite of the albedo transition process like space weathering, the heliocentric distribution of the mean albedo of asteroids in each taxonomic type is found to be nearly flat. In the total distribution, on the other hand, the mean albedo value gradually decreases with increasing the semimajor axis, presumably due to the compositional mixing ratios of taxonomic types. In this talk, we present the details of data compiling of size, albedo, and taxonomy of MBAs, and discuss the compositional distribution in the main belt regions.