IM Lup周围年轻行星形成盘中亚微米尺寸颗粒的分形聚集
Fractal aggregates of sub-micron-sized grains in the young planet-forming disk around IM Lup
摘要：
尽管对星盘的观测迅速增长，但原始尘埃聚集体的样子以及它们的组成颗粒（单体）在年轻的行星形成星盘中的物理和化学性质仍然是一个谜。将模型与观测结果对抗来回答这个谜团一直是一项臭名昭著的任务，因为我们必须放弃一个常用的假设，即完美的球形颗粒，并考虑具有复杂形态的粒子。在这封信中，我们首次对IM Lup周围年轻行星形成盘的近红外散射光与复杂形状尘埃颗粒的光散射特性进行了彻底的比较。在多个波长和散射角的显着范围内的散射观测的可用性允许的单体大小，分形维数，并在行星形成盘的尘埃聚集体的大小的第一个确定。我们发现，观察到的最好的解释分形聚集体的分形维数为1.5和大于$\sim2~μ$ m的特征半径。我们还确定了单体的半径为$\sim200$ nm，在分形维数小于2的前提下，可以排除比这个尺寸小得多的单体。此外，尘埃组合物包括无定形碳被发现是有利的，同时考虑到微弱的散射光和耀斑盘形态。我们的研究结果支持行星形成开始于亚微米级颗粒的分形凝聚。在这项研究中计算的复杂尘埃粒子的所有光学特性都是公开的。
英文摘要：
Despite rapidly growing disk observations, it remains a mystery what primordial dust aggregates look like and what the physical and chemical properties of their constituent grains (monomers) are in young planet-forming disks. Confrontation of models with observations to answer this mystery has been a notorious task because we have to abandon a commonly used assumption, perfectly spherical grains, and take into account particles with complex morphology. In this Letter, we present the first thorough comparison between near-infrared scattered light of the young planet-forming disk around IM Lup and the light-scattering properties of complex-shaped dust particles. The availability of scattering observations at multiple wavelengths and over a significant range of scattering angles allows for the first determination of the monomer size, fractal dimension, and size of dust aggregates in a planet-forming disk. We show that the observations are best explained by fractal aggregates with a fractal dimension of 1.5 and a characteristic radius larger than $\sim2~μ$m. We also determined the radius of the monomer to be $\sim200$ nm, and monomers much smaller than this size can be ruled out on the premise that the fractal dimension is less than 2. Furthermore, dust composition comprising amorphous carbon is found to be favorable to simultaneously account for the faint scattered light and the flared disk morphology. Our results support that planet formation begins with fractal coagulation of sub-micron-sized grains. All the optical properties of complex dust particles computed in this study are publicly available.
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