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. 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 cur ve 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 emiss ion from these asteroids spanning the infrared and radio ranges. The e ffective emissivity e(EFF) is defined as the ratio of the observed flu x density to that which would have been observed from a nonrotating, s pherical blackbody with the same size, distance from the Earth, and di stance from the Sun, as though viewed at opposition. The physical prop erties that influence e(EFF) are discussed qualitatively, using our SE Ds to illustrate the importance of each effect. In this way, the effec tive emissivity is demonstrated to be a useful means to present the SE D of an asteroid over the whole range of wavelengths for which thermal emission dominates the observable flux density. The most important ph ysical properties that distinguish the SEDs of the nonmetallic asteroi ds (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 densit y of the surface materials; and (3) the rotation period of the asteroi d. For Ceres the warm surface layer is partially opaque at wavelengths near 1 mm, while for Vesta it is transparent at all wavelengths longe r 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 i ts surface to its deeper layers, reducing the infrared beaming compare d to Ceres and increasing the optical depth of its warm surface layer compared to Vesta. The effectiveness of rotation in suppressing the in frared beaming phenomenon is illustrated by Vesta, a rapid rotator wit h a weak infrared beaming effect, and by Melpomene, a slow rotator wit h a strong infrared beaming effect. The SEDs of the M-type (metallic) asteroids (Psyche and Kleopatra) have a distinctive shape, with a stee p decrease from infrared to radio wavelengths. The effective emissivit ies at wavelengths near 1 mm are too low to correspond to physical tem peratures in the asteroids' surfaces but are consistent with the prese nce of large metal fractions in their surface minerals, which would ma ke their surfaces reflective rather than emissive at long wavelengths. This is the first clear mineralogical distinction that we have been a ble to make based on the shape of an asteroid's SED.