Physical Model of Dust Polarization by Radiative Torque Alignment and Disruption and Implications for Grain Internal Structures
Physical Model of Dust Polarization by Radiative Torque Alignment and Disruption and Implications for Grain Internal Structures
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DOI：
10.3847/1538-4357/ab8e33
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发表时间：
2019-11
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影响因子：
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通讯作者：
Hyeseung Lee;Thiem C. Hoang;Ngân Lê;Jungyeon Cho
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作者：
Hyeseung Lee;Thiem C. Hoang;Ngân Lê;Jungyeon Cho
尘埃极化取决于尘埃的机械性质以及局部环境。为了了解尘埃偏振如何随不同的属性而变化，我们通过同时考虑晶粒排列和辐射扭矩（RAT）的旋转破坏来模拟星光的波长依赖性偏振和对齐晶粒的偏振尘埃发射。我们探索了广泛的局部辐射场和晶粒的力学性能，其特征在于拉伸强度（Smax）。我们发现，随着辐射强度（U）的增加，由于小晶粒的排列增强，峰值波长向更短的波长移动。RATs的颗粒旋转破坏倾向于降低光学近红外偏振，但增加星光的紫外偏振，由于大颗粒转化为较小的。特别地，我们发现850 μm处的极化度（P850）并不随U或晶粒温度（Td）单调增加，而是取决于晶粒的Smax。我们的模型可以通过对恒星形成区或分子云的观测来检验，这些区域或分子云被附近的星星照射，其辐射强度比平均星际辐射场的辐射强度高。最后，我们比较了我们的预测P850-Td的关系与普朗克数据，发现所观察到的减少P850与Td可以解释时，由RAT颗粒中断占，这表明星际颗粒不太可能有一个紧凑的结构，也许他们有一个复合的。偏振度随U（或Td）的变化可以对宇宙尘埃的内部结构提供有价值的约束。
Dust polarization depends on mechanical properties of dust as well as on local environments. To understand how dust polarization varies with different properties, we model the wavelength-dependence polarization of starlight and polarized dust emission of aligned grains by simultaneously taking into account grain alignment and rotational disruption by radiative torques (RATs). We explore a wide range of the local radiation field and grain mechanical properties characterized by tensile strength (Smax). We find that the peak wavelength shifts to shorter wavelengths as the radiation strength (U) increases due to the enhanced alignment of small grains. Grain rotational disruption by RATs tends to decrease the optical-NIR polarization but increase the UV polarization of starlight due to the conversion of large grains into smaller ones. In particular, we find that the polarization degree at 850 μm (P850) does not increase monotonically with U or grain temperature (Td), but it depends on Smax of the grains. Our model can be tested with observations toward star-forming regions or molecular clouds irradiated by a nearby star, which have higher radiation intensities than the that of the average interstellar radiation field. Finally, we compare our predictions of the P850–Td relationship with Planck data and find that the observed decrease of P850 with Td can be explained when grain disruption by RATs is accounted for, suggesting that as interstellar grains are unlikely to have a compact structure, perhaps they have a composite one. The variation of the polarization degree with U (or Td) can provide a valuable constraint on the internal structure of cosmic dust.