论文标题
在MIE制度中，对齐晶粒的极化发射：Alma观察到的原月球磁盘的应用
Polarized emission by aligned grains in the Mie regime : application to protoplanetary disks observed by ALMA
论文作者
论文摘要
Alma在毫米波长下在某些原始球星盘中观察到的方位角极化模式提出了对它们与磁场线排列的防尘粒所产生的怀疑。这些结论是基于雷利策略中灰尘极化发射的计算，即，对于比波长小得多的晶粒大小。但是，估计此类磁盘中的晶粒尺寸通常位于独立观测值的0.1-1 mm范围内。我们研究了MIE机制中排列晶粒在发射中的灰尘极化特性，即平均晶粒尺寸分配波长时。使用t-matrix和dustem代码，我们计算出完美旋转对齐晶粒发射中极化部分的光谱依赖性，用于弱延伸的植物尺寸分布，弱延伸的植物和天体硅酸盐组成的晶粒，平均大小为10μm至1 mm。在亚毫米和毫米波长范围内，B场对齐晶粒的极化对于大于〜250μm的晶粒而言是负的，这意味着极化向量与B场平行。从正极到负极化的过渡发生在波长λ〜1 mm处。对于小于〜100μm的晶粒不存在负极化的状态。当使用晶粒尺寸至亚毫升尺寸的磁盘上使用逼真的晶粒尺寸分布时，对齐晶粒的热发射的极化方向与磁场的方向平行于通常用于观察年轻原始磁盘的大量波长。该特性可以解释在ALMA观察到的某些磁盘中极化向量的特殊方位角方向，并证明了灰尘偏振发射在磁盘中追踪磁场的保守能力。
The azimuthal polarization patterns observed in some protoplanetary disks by ALMA at millimeter wavelength have raised doubts about their being produced by dust grains aligned with the magnetic field lines. These conclusions were based on the calculations of dust polarized emission in the Rayleigh regime, i.e. for grain sizes much smaller than the wavelength. However, the grain size in such disks is estimated to be typically in the range 0.1 - 1 mm from independent observations. We study the dust polarization properties of aligned grains in emission in the Mie regime, i.e. when the mean grain size approches the wavelength. Using the T-MATRIX and DustEM codes, we compute the spectral dependence of the polarization fraction in emission for grains in perfect spinning alignment, for various grain size distributions of weakly-elongated oblate and prolate grains of astrosilicate composition, with a mean size ranging from 10 μm to 1 mm. In the submillimeter and millimeter wavelength range, the polarization by B-field aligned grains becomes negative for grains larger than ~ 250 μm, meaning that the polarization vector becomes parallel to the B-field. The transition from the positive to the negative polarization occurs at a wavelength λ ~ 1 mm. The regime of negative polarization does not exist for grains smaller than ~ 100 μm. When using realistic grain size distributions for disks with grains up to the submillimeter sizes, the polarization direction of thermal emission by aligned grains is shown to be parallel to the direction of the magnetic field over a significant fraction of the wavelengths typically used to observe young protoplanetary disks. This property may explain the peculiar azimuthal orientation of the polarization vectors in some of the disks observed with ALMA and attest of the conserved ability of dust polarized emission to trace the magnetic field in disks.
