Interpretation of Herschel's "cold" debris disks

赫歇尔开放时间关键计划沙丘(附近恒星周围的尘埃)的一个重要结果是发现了一类新的“冷”碎片盘。这些是围绕在主序恒星周围的细小尘埃盘，在100μm处显示很少或没有红外过剩，但在160μm和可能更长的波长上显示出显著的红外过剩。从光谱能量分布推断的尘埃温度与从分辨率图像估计的圆盘半径的比较表明，尘埃比尘埃位置的黑体更冷。这就要求颗粒要比远红外波长大，在可见光下的吸收也要低。后者是可以实现的，例如，如果尘埃中含有丰富的冰，我们将通过假设发射物质的各种材料成分来研究这种可能性。乍一看，没有小颗粒更令人费解，因为碎片盘的标准碰撞模型预测，存在各种大小的颗粒，小到辐射压力喷发极限的几倍。然而，有几种可以想象的情景来解释小颗粒的枯竭：以运输为主的圆盘，具有动态冷尘埃产生的行星体的圆盘，以及未搅拌的原始毫米大小颗粒的圆盘。这些情况并不是相互排斥的，将进行深入调查。简而言之，该项目的主要目标是找到冷碎片盘的解释。如果被发现，它将有助于揭示类太阳恒星周围小行星形成的环境和效率，并最终有助于更好地理解行星系统的形成和演化。

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.