[论文解读] Self-scattering of non-spherical dust grains. The limitations of perfect compact spheres

Florian Kirchschlager, Gesa H.-M. Bertrang|arXiv (Cornell University)|Jun 1, 2020

Astrophysics and Star Formation Studies参考文献 28被引用 6

一句话总结

本研究挑战了自散射模型中尘埃颗粒为完美致密球体的假设，表明非球形颗粒（尤其是扁球形颗粒）在亚毫米波段（>100 μm）产生的偏振度显著高于球形颗粒。结果表明，球形模型会系统性低估偏振度，并可能导致尘埃尺寸估计出现一个数量级的偏差，因此必须重新评估基于ALMA观测的当前尘埃尺寸估计。

ABSTRACT

Context. The understanding of (sub-)millimetre polarisation has made a leap forward since high-resolution imaging with the Atacama Large (sub-)Mm Array (ALMA) became available. Amongst other effects, self-scattering (i.e. the scattering of thermal dust emission on other grains) is thought to be the origin of millimetre polarisation. This opens the first window to a direct measurement of dust grain sizes in regions of optically thick continuum emission as it can be found in protoplanetary discs and star-forming regions. However, the newly derived values of grain sizes are usually around ~100 μm and thus one order of magnitude smaller than those obtained from more indirect measurements, as well as those expected from theory (~1 mm). /
 
 Aims. We see the origin of this contradiction in the applied dust model of current self-scattering simulations: a perfect compact sphere. The aim of this study is to test our hypothesis by investigating the impact of non-spherical grain shapes on the self-scattering signal. /
 
 Methods. We applied discrete dipole approximation simulations to investigate the influence of the grain shape on self-scattering polarisation in three scenarios: an unpolarised and polarised incoming wave under a fixed and a varying incident polarisation angle. /
 
 Results. We find significant deviations of the resulting self-scattering polarisation when comparing non-spherical to spherical grains. In particular, tremendous deviations are found for the polarisation signal of grains when observed outside the Rayleigh regime, that is for >100 μm sized grains observed at the 870 μm wavelength. Self-scattering by oblate grains produces higher polarisation degrees compared to spheres, which challenges the interpretation of the origin of observed millimetre polarisation. A (nearly) perfect alignment of the non-spherical grains is required to account for the observed millimetre polarisation in protoplanetary discs. Furthermore, we find conditions under which the emerging scattering polarisation of non-spherical grains is flipped in orientation by 90°. /
 
 Conclusions. These results show clearly that the perfect compact sphere is an oversimplified model, which has reached its limit. Our findings point towards a necessary re-evaluation of the dust grain sizes derived from (sub-)millimetre polarisation.

研究动机与目标

• 研究非球形尘埃颗粒形状对（亚）毫米波段自散射偏振的影响。
• 解决观测到的亚毫米波段偏振与理论/间接粒径估计（约1 mm vs. 约100 μm）之间的差异。
• 检验自散射模拟中假设完美致密球体是否导致粒径推断的系统性偏差。
• 评估颗粒形状如何影响偏振度与位置角，特别是在瑞利近似以外的区域。

提出的方法

• 采用离散偶极近似（DDA）模拟来建模非球形尘埃颗粒的散射行为。
• 模拟了三种情形：未偏振和偏振入射波，且入射偏振角固定或变化。
• 颗粒形状包括扁球形、长球形和球形，以比较其散射特性。
• 针对870 μm波长下大于100 μm的颗粒，计算了偏振度与位置角。
• 模拟聚焦于颗粒尺寸大于观测波长的区域，超出瑞利近似适用范围。
• 将结果与标准球形颗粒模型进行对比，量化偏振响应的偏差。

实验结果

研究问题

• RQ1 非球形尘埃颗粒形状如何影响亚毫米波段自散射偏振的偏振度与取向？
• RQ2 在非瑞利区域，扁球形颗粒相比球形颗粒在多大程度上增强偏振度？
• RQ3 在合理的颗粒对齐条件下，非球形颗粒能否再现原行星盘中观测到的亚毫米波段偏振？
• RQ4 在何种条件下，非球形颗粒模型中散射光的偏振位置角会发生90°翻转？
• RQ5 将颗粒假设为球形会如何系统性地偏差从ALMA偏振数据推断出的尘埃颗粒尺寸？

主要发现

• 非球形颗粒，特别是扁球形颗粒，在约870 μm波长及粒径>100 μm时产生的偏振度显著高于球形颗粒。
• 在非瑞利区域，扁球形颗粒的偏振度可远超球形颗粒，尤其在大颗粒区域。
• 为再现观测到的亚毫米波段偏振水平，需近乎完美的颗粒对齐。
• 在某些几何构型下，由于颗粒形状各向异性，散射光的偏振位置角会发生90°翻转。
• 标准假设中完美致密球体导致偏振度被系统性低估，从而使得基于此类模型推断的当前尘埃粒径估计无效。
• 这些发现表明，目前从（亚）毫米波段偏振推断出的尘埃粒径（通常约100 μm）很可能低估了一个数量级。