Multiple asteroid systems: Dimensions and thermal properties from Spitzer Space Telescope and ground-based observations

We collected mid-IR spectra from 5.2 to 38 mu m using the Spitzer Space Telescope Infrared Spectrograph of 28 asteroids representative of all established types of binary groups. Photometric lightcurves were also obtained for 14 of them during the Spitzer observations to provide the context of the observations and reliable estimates of their absolute magnitudes. The extracted mid-IR spectra were analyzed using a modified standard thermal model (STM) and a thermophysical model (TPM) that takes into account the shape and geometry of the large primary at the time of the Spitzer observation. We derived a reliable estimate of the size, albedo, and beaming factor for each of these asteroids, representing three main taxonomic groups: C, S. and X. For large (volume-equivalent system diameter D-eq > 130 km) binary asteroids, the TPM analysis indicates a low thermal inertia (Gamma <= similar to 100 J s(-1/2) K-1 m(-2)) and their emissivity spectra display strong mineral features, implying that they are covered with a thick layer of thermally insulating regolith. The smaller (surface-equivalent system diameter D-eff < 17 km) asteroids also show some emission lines of minerals, but they are significantly weaker, consistent with regoliths with coarser grains, than those of the large binary asteroids. The average bulk densities of these multiple asteroids vary from 0.7-1.7 g/cm(3) (P-, C-type) to similar to 2 g/cm(3) (S-type). The highest density is estimated for the M-type (22) Kalliope (3.2 +/- 0.9 g/cm(3)). The spectral energy distributions (SEDs) and emissivity spectra, made available as a supplement document, could help to constrain the surface compositions of these asteroids. (C) 2012 Elsevier Inc. All rights reserved.