标题： 厚HI介质中的Lyα偏振 显示英文标题

标题： Lya Polarisation in Thick H I Media

摘要： 我们研究了在静态、无尘且厚的平面平行薄片中传输的氢原子中性Lya的线偏振特性，该薄片均匀充满中性氢。 由于散射相函数对于翼区散射表现出偶极型特征，这与共振散射相关的更各向同性的散射相函数形成鲜明对比，因此出射Lya的偏振主要由逃逸前的翼区散射次数控制。 当散射介质极其厚时，以掠入射方向出射的光子偏振方向垂直于薄片法线，最高可达12%，这一结果与钱德拉塞卡在1960年研究的半无限厚介质中汤姆逊散射辐射达到的极限一致。 逃逸前的翼区散射次数对Voigt参数 $a$和线中心光学深度 $\tau_0$的乘积非常敏感，使用这些参数也可以解释偏振度的变化与汤姆逊散射情况的对比。 当 $a\tau_0<10^3$时，偏振方向平行于薄片法线；而当 $a\tau_0>10^3$时，偏振方向垂直于薄片法线。 我们模拟的来自厚介质的Lya的光谱偏振特性表现为：线中心附近的偏振几乎可以忽略，近翼部分平行偏振，远翼部分垂直偏振，偏振方向相对于薄片法线测量。 偏振反转自然反映了Lya传输过程在实空间和频率空间中的扩散性质。 我们还讨论了与发射近翼部分平行偏振相关的束流效应。 显示英文摘要

摘要： We have investigated the linear polarisation of Lya transferred in a static, dustless, and thick plane-parallel slab that is uniformly filled with neutral hydrogen. Because the scattering phase function is characterised by the dipole type for wing scatterings of Lya, which is in high contrast with more isotropic scattering phase function associated with resonant scatterings, the polarisation of the emergent Lya is dominantly controlled by the number of wing scatterings before escape from the slab. When the scattering medium is extremely thick, photons emergent in the grazing direction are polarised in the direction perpendicular to the slab normal up to 12%, which coincides with the limit attained by the Thomson scattered radiation in a semi-infinite thick medium investigated by Chandrasekhar in 1960. The number of wing scatterings before escape is sensitively dependent on the product of the Voigt parameter $a$ and the line-centre optical depth $\tau_0$, with which the variation of the degree of polarisation can also be explained in comparison with the Thomson scattering case. The polarisation direction is parallel to the slab normal when $a\tau_0<10^3$, while it is perpendicular when $a\tau_0>10^3$. Our simulated spectropolarimetry of Lya from a highly thick medium is characterised by negligible polarisation around the line centre, parallel polarisation in the near wing parts and perpendicular polarisation in the far wing parts, where the polarisation direction is measured with respect to the slab normal. The polarisation flip naturally reflects the diffusive nature of the Lya transfer process in both real space and frequency space. We also discuss the beaming effect that is associated with the parallel polarisation in the near wing parts of the emission.