Thermal emission and scattering by aligned grains: Plane-parallel model and application to multiwavelength polarization of the HL Tau disc
Thermal emission and scattering by aligned grains: Plane-parallel model and application to multiwavelength polarization of the HL Tau disc
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对齐颗粒的热发射和散射：平面平行模型及其在 HL Tau 圆盘多波长偏振中的应用
DOI：
10.1093/mnras/stac753
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发表时间：
2022
影响因子：
4.8
通讯作者：
Fernández-López, Manuel
中科院分区：
文献类型：
--
作者：
Lin, Zhe-Yu Daniel;Li, Zhi-Yun;Yang, Haifeng;Stephens, Ian;Looney, Leslie;Harrison, Rachel;Fernández-López, Manuel
望远镜现在能够在多个波长上分辨星周圆盘上的尘埃偏振，从而可以研究偏振光谱。大多数星盘都清楚地显示出尘埃通过其单向偏振模式散射的证据，通常是在波长较短的波段。然而，某些圆盘在~ 3mm处显示椭圆形图案，这可能是由于排列的颗粒。对于HL Tau，其在~ 1.3 mm处的偏振模式显示出两种模式之间的转变，使其成为揭示这种转变的第一个例子。本文采用t矩阵法对细长粉尘颗粒进行了模拟，并采用平行平板模型对排列的非球形粉尘颗粒散射进行了适当的处理。我们证明了光深的变化可以很自然地解释HL Tau的偏振跃迁。在低光深处，热偏振占主导地位，而在高光深处，二向色消光有效地消除了热偏振，散射偏振占主导地位。在平面平行平板实验结果的启发下，我们开发了一种简单的技术，利用偏振分数的方位角变化来解开排列晶粒的热偏振和散射引起的偏振。我们发现，随着波长的增加，散射分量的分数偏振谱减小，而热分量的分数偏振谱增加，这是由于光学深度减小而预期的。我们发现了几个类似于HL Tau的其他来源，当包括光学深度效应时，可以用方位排列散射延长颗粒来解释。此外，我们探讨了螺旋排列的散射颗粒如何出现在偏振图像中。
Telescopes are now able to resolve dust polarization across circumstellar discs at multiple wavelengths, allowing the study of the polarization spectrum. Most discs show clear evidence of dust scattering through their unidirectional polarization pattern typically at the shorter wavelength ofm. However, certain discs show an elliptical pattern at ∼3 mm, which is likely due to aligned grains. With HL Tau, its polarization pattern at ∼1.3 mm shows a transition between the two patterns making it the first example to reveal such transition. We use the T-matrix method to model elongated dust grains and properly treat scattering of aligned non-spherical grains with a plane-parallel slab model. We demonstrate that a change in optical depth can naturally explain the polarization transition of HL Tau. At low optical depths, the thermal polarization dominates, while at high optical depths, dichroic extinction effectively takes out the thermal polarization and scattering polarization dominates. Motivated by results from the plane-parallel slab, we develop a simple technique to disentangle thermal polarization of the aligned grainsT0and polarization due to scatteringSusing the azimuthal variation of the polarization fraction. We find that, with increasing wavelength, the fractional polarization spectrum of the scattering componentSdecreases, while the thermal componentT0increases, which is expected since the optical depth decreases. We find several other sources similar to HL Tau that can be explained by azimuthally aligned scattering prolate grains when including optical depth effects. In addition, we explore how spirally aligned grains with scattering can appear in polarization images.