Interpretation of Herschel's "cold" debris disks

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An important result of the Herschel Open Time Key Program DUNES (DUst around Nearby Stars) is the discovery of a new class of “cold” debris disks. These are tenuous dust disks around main-sequence stars that show little or no infrared excess at 100 μm, but a significant one at 160 μm and possibly longer wavelengths. A comparison of the dust temperatures inferred from the spectral energy distributions with the disk radii estimated from the resolved images suggests that the dust is colder than a black body at the dust location. This requires the grains to be large compared to far-infrared wavelengths and to have low absorption in the visible. The latter could be achieved, for instance, if the dust is rich in ices, and we will study the possibilities by assuming various material compositions of the emitting material. The absence of small grains is, at a first glance, more puzzling, since standard collisional models of debris disks predict the grains of all sizes down to several times the radiation pressure blowout limit to be present. However, there exist several conceivable scenarios to explain depletion of small grains: transport-dominated disks, disks with dynamically cold dust-producing planetesimals, and the disks of unstirred primordial millimeter-sized grains. These scenarios, which are not mutually exclusive, will be investigated in depth. In short, the main goal of the project is to find an explanation for the cold debris disks. If found, it would shed light onto the circumstances and efficiency of planetesimal formation around Sun-like stars and ultimately, help better understand the formation and evolution of planetary systems.