arXiv 2001.084002020-01-23astro-ph.GA

米氏体系中排列晶粒的偏振发射：应用于ALMA观测的原行星盘

Polarized emission by aligned grains in the Mie regime : application to protoplanetary disks observed by ALMA

V. Guillet, J. M. Girart, A. J. Maury, F. O. Alves

发表信息：: A&A 634, L15 (2020)
DOI：: 10.1051/0004-6361/201937314

摘要：

ALMA在毫米波段观测到的某些原行星盘的方位角偏振模式，使人们怀疑它们是由与磁场线对齐的尘埃颗粒产生的。这些结论是基于瑞利机制中尘埃偏振发射的计算，即颗粒尺寸远小于波长。然而，在这样的磁盘的晶粒尺寸估计通常是在0.1 - 1毫米的范围内，从独立的观察。本文研究了在米氏区，即平均晶粒尺寸接近波长时，定向晶粒发射的尘埃偏振特性。使用T-MATRIX和DustEM程序，我们计算了在完全自旋排列的晶粒中，对于不同粒度分布的弱伸长扁长和长扁星硅酸盐组成的晶粒，平均粒度从10 μm到1 mm，发射偏振分数的光谱依赖性。在亚毫米和毫米波长范围内，对于大于~ 250 μm的晶粒，B场取向晶粒的极化变为负，这意味着极化矢量变为平行于B场。在波长λ ~ 1 mm处出现从正极化到负极化的转变，小于100 μm的晶粒不存在负极化区。当使用现实的晶粒尺寸分布的磁盘与晶粒的亚毫米级的大小，由对齐的晶粒的热发射的偏振方向被证明是平行的磁场的方向在一个显着的分数通常用于观察年轻的原行星盘的波长。这一性质可以解释ALMA观测到的某些圆盘中极化矢量的特殊方位角取向，并证明了尘埃极化辐射追踪圆盘中磁场的守恒能力。

英文摘要：

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.

机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。

摘要：

英文摘要：

相关论文