High-Quality Photometry of Asteroids at Millimeter and Submillimeter Wavelengths

Abstract

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. The measurements reported here include a careful estimation of the possible systematic biases that may be present in the photometry. Whenever possible, the flux densities have been averaged over a complete rotational light curve to eliminate rotational phase as a source of uncertainty. Combining our measurements with those from the literature at other wavelengths, we present spectral energy distributions (SEDs) for the thermal emission from these asteroids spanning the infrared and radio ranges. The effective emissivity eEFF 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 eEFF 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 (Ceres, Vesta, Hebe, Iris, and Melpomene) 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. We attribute the transparency of Vesta's warm surface layer to its low density. In contrast, Iris appears to have relatively dense materials on its surface that transport heat effectively from its surface to its deeper layers, reducing the infrared beaming compared to Ceres and increasing the optical depth of its warm surface layer compared to Vesta. The effectiveness of rotation in suppressing the infrared beaming phenomenon is illustrated by Vesta, a rapid rotator with a weak infrared beaming effect, and by Melpomene, a slow rotator with a strong infrared beaming effect. The SEDs of the M-type (metallic) asteroids (Psyche and Kleopatra) have a distinctive shape, with a steep decrease from infrared to radio wavelengths. The effective emissivities at wavelengths near 1 mm are too low to correspond to physical temperatures in the asteroids' surfaces but are consistent with the presence of large metal fractions in their surface minerals, which would make their surfaces reflective rather than emissive at long wavelengths. This is the first clear mineralogical distinction that we have been able to make based on the shape of an asteroid's SED.