利用偏振研究颗粒取向与尘埃演化物理（GRADE-POL）I. 孤立无恒星核的尘埃偏振建模
Grain alignment and dust evolution physics with polarisation (GRADE-POL). I. Dust polarisation modelling for isolated starless cores
AI总结：
本研究基于磁增强辐射力矩（MRAT）理论开展孤立无恒星核的尘埃偏振数值建模，解释了偏振洞现象，成功复现多波段观测结果，为各向异性颗粒生长新模型提供了证据。
摘要：
由取向星际尘埃引发的光偏振是研究宇宙磁场、尘埃性质的重要工具，同时也给表征宇宙微波背景及其他源的偏振特性带来了挑战。要将尘埃偏振确立为可靠的研究工具，需深入探究颗粒取向过程的物理机制。磁增强辐射力矩（MRAT）取向是唯一能实现颗粒与磁场高效取向的机制，而这种高效取向是弥漫星际介质偏振观测所必需的。我们基于MRAT理论开展的尘埃偏振数值模拟表明，随着可见光消光值（$A_{\rm V}$）升高，恒星光偏振效率（$p_{\rm ext}/A_{\rm V}$）与热尘埃偏振度（$p_{\rm em}$）先缓慢下降，在$A_{\rm V}$较大时因颗粒取向失效呈$\propto A^{-1}_{\rm V}$规律陡降，这一结果解释了被称为偏振洞的现象。由缓降转为陡降的过渡消光值（$A^{\rm loss}_{\rm V}$）随颗粒最大尺寸增大而升高。我们采用适用于烟斗星云（Pipe Nebula）中无恒星核109（Pipe-109）的物理剖面开展建模，成功复现了现有R波段（$0.65\\,\mu$m）、H波段（$1.65\\,\mu$m）恒星光偏振观测结果，以及亚毫米波段（$870\\,\mu$m）的发射偏振观测结果。观测数据的成功建模需要大颗粒完美取向作为MRAT机制的证据，同时要求高$A_{\rm V}$区域存在更大最大尺寸、更高延展率的颗粒，后者首次为磁场诱导颗粒取向引发的各向异性颗粒生长新模型提供了证据。
英文摘要：
The polarisation of light induced by aligned interstellar dust serves as a significant tool in investigating cosmic magnetic fields, dust properties, and poses a challenge in characterising the polarisation of the cosmic microwave background and other sources. To establish dust polarisation as a reliable tool, the physics of the grain alignment process needs to be studied thoroughly. The Magnetically enhanced Radiative Torque (MRAT) alignment is the only mechanism that can induce highly efficient alignment of grains with magnetic fields required by polarisation observations of the diffuse interstellar medium. Our numerical modelling of dust polarisation using the MRAT theory demonstrated that the alignment efficiency of starlight polarisation ($p_{\rm ext}/A_{\rm V}$) and the degree of thermal dust polarisation ($p_{\rm em}$) first decrease slowly with increasing visual extinction ($A_{\rm V}$) and then falls steeply as $\propto A^{-1}_{\rm V}$ at large $A_{\rm V}$ due to the loss of grain alignment, which explains the phenomenon known as polarisation holes. Visual extinction at the transition from shallow to steep slope ($A^{\rm loss}_{\rm V}$) increases with the maximum grain size. By applying physical profiles suitable for a starless core 109 in the Pipe Nebula (Pipe-109), our model successfully reproduces the existing observations of starlight polarisation at R-band ($0.65\,μ$m) and H-band ($1.65\,μ$m), as well as emission polarisation at submillimetre ($870\,μ$m). Successful modelling of observational data requires perfect alignment of large grains as evidence of the MRAT mechanism, and larger maximum size with higher elongation at higher $A_{\rm V}$. The latter reveals the first evidence for the new model of anisotropic grain growth induced by magnetic grain alignment.
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