2016ApJ...831...97M

We present dual-band Herschel/PACS imaging for 59 main-sequence stars with known warm dust (T_warm ∼ 200 K), characterized by Spitzer. Of 57 debris disks detected at Herschel wavelengths (70 and/or 100 and 160 µm), about half have spectral energy distributions (SEDs) that suggest two-ring disk architectures mirroring that of the asteroid-Kuiper Belt geometry; the rest are consistent with single belts of warm, asteroidal material. Herschel observations spatially resolve the outer/cold dust component around 14 A-type and 4 solar-type stars with two-belt systems, 15 of which for the first time. Resolved disks are typically observed with radii >100 AU, larger than expected from a simple blackbody fit. Despite the absence of narrow spectral features for ice, we find that the shape of the continuum, combined with resolved outer/cold dust locations, can help constrain the grain size distribution and hint at the dust's composition for each resolved system. Based on the combined Spitzer/IRS+Multiband Imaging Photometer (5-to-70 µm) and Herschel/PACS (70-to-160 µm) data set, and under the assumption of idealized spherical grains, we find that over half of resolved outer/cold belts are best fit with a mixed ice/rock composition. Minimum grain sizes are most often equal to the expected radiative blowout limit, regardless of composition. Three of four resolved systems around the solar-type stars, however, tend to have larger minimum grains compared to expectation from blowout (f_MB = a_min/a_BOS ∼ 5). We also probe the disk architecture of 39 Herschel-unresolved systems by modeling their SEDs uniformly, and find them to be consistent with 31 single- and 8 two-belt debris systems.