Validating scattering-induced (sub)millimetre disc polarization through the spectral index, wavelength-dependent polarization pattern, and polarization spectrum: the case of HD 163296
Validating scattering-induced (sub)millimetre disc polarization through the spectral index, wavelength-dependent polarization pattern, and polarization spectrum: the case of HD 163296
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DOI：
10.1093/mnras/staa1499
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发表时间：
2019-12
影响因子：
4.8
通讯作者：
Zheyu Lin;Zhi-Yun Li;Haifeng Yang;L. Looney;I. Stephens;C. Hull
中科院分区：
文献类型：
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作者：
Zheyu Lin;Zhi-Yun Li;Haifeng Yang;L. Looney;I. Stephens;C. Hull
许多年轻的环绕恒星的盘沿短轴显示出惊人的有序(亚)毫米偏振方向，这是0.1毫米大小的∼颗粒散射导致的偏振的有力证据。为了进一步检验这一机制，我们用RADMC-3D模拟了高清163296的ALMA尘埃连续谱和偏振数据。我们发现，最大尺寸为90μm的颗粒的散射同时再现了在7带观察到的偏振和光学厚度内盘中7和6带之间的异常低的光谱折射率(α∼1.5)，这是由于在较短波长下的更有效散射的结果。为光学薄的带隙推断的低光谱指数∼2.5是由相同的颗粒再现的，这是望远镜对带隙(具有固有的α∼4)及其邻近的光学厚环(其中α≲2)进行平均的结果。从偏振和光谱折射率推断的颗粒尺寸之间的紧张消失了，因为低α值不需要大毫米尺寸的颗粒。此外，偏振分数具有独特的方位变化：在缝隙中沿长轴的高度高于短轴，而在环中则相反。我们在带隙中发现了一个快速衰减的偏振谱(p∝λ−约为3)，在光学厚度较厚的环中，它变得平坦甚至向短波长反转。环和缝隙中的这些对比行为为通过分辨的多波长观测来测试散射激发极化提供了进一步的测试。
A number of young circumstellar discs show strikingly ordered (sub)millimetre polarization orientations along the minor axis, which is strong evidence for polarization due to scattering by ∼0.1 mm-sized grains. To test this mechanism further, we model the ALMA dust continuum and polarization data of HD 163296 using radmc-3d. We find that scattering by grains with a maximum size of 90 μm simultaneously reproduces the polarization observed at Band 7 and the unusually low spectral index (α ∼ 1.5) between Bands 7 and 6 in the optically thick inner disc as a result of more efficient scattering at the shorter wavelength. The low spectral index of ∼2.5 inferred for the optically thin gaps is reproduced by the same grains, as a result of telescope beam averaging of the gaps (with an intrinsic α ∼ 4) and their adjacent optically thick rings (where α ≲ 2). The tension between the grain sizes inferred from polarization and spectral index disappears because the low α values do not require large mm-sized grains. In addition, the polarization fraction has a unique azimuthal variation: higher along the major axis than the minor axis in the gaps, but vice versa in the rings. We find a rapidly declining polarization spectrum (with p ∝ λ−3 approximately) in the gaps, which becomes flattened or even inverted towards short wavelengths in the optically thick rings. These contrasting behaviours in the rings and gaps provide further tests for scattering-induced polarization via resolved multiwavelength observations.