High-Quality Photometry of Asteroids at Millimeter and Submillimeter Wavelengths

Photometric observations with the JCMT at millimeter and submillimeter wavelengths have been made for the asteroids 1 Ceres, 4 Vesta, 6 Hebe, 7 Iris, 16 Psyche, 18 Melpomene, and 216 Kleopatra. We present spectral energy distributions (SEDs) for the thermal emission from these asteroids. The effective emissivity e(EFF) is defined as the ratio of the observed flux density to that which would have been observed from a nonrotating, spherical blackbody with the same size, distance from the Earth, and distance from the sun, as though viewed at opposition. The physical properties that influence e(EFF) are discussed qualitatively, using our SEDs to illustrate the importance of each effect. In this way, the effective emissivity is demonstrated to be a useful means to present the SED of an asteroid over the whole range of wavelengths for which thermal emission dominates the observable flux density. The most important physical properties that distinguish the SEDs of the nonmetallic asteroids from each other appear to be (1) the optical depth through the layer of warm material that has been heated by the sun on the day side of the asteroid; (2) the density of the surface materials; and (3) the rotation period of the asteroid. For Ceres the warm surface layer is partially opaque at wavelengths near 1 mm, while for Vesta it is transparent at all wavelengths longer than 0.35 mm. Iris appears to have relatively dense materials on its surface that transport heat effectively from its surface to its deeper layers. (Author)