尘埃对光学厚晕中莱曼α光子传输的影响
Effect of Dust on Lyman-alpha Photon Transfer in Optically Thick Halo
摘要：
通过求解共振光子辐射转移的积分微分方程，研究了尘埃对光学厚介质中Lyα光子出射的影响.为了数值求解微分方程，我们使用加权基本无振荡方法（WENO）。虽然尘埃对辐射传输的影响是众所周知的，但Lyα光子的共振散射使问题变得不平凡。例如，如果介质的灰尘吸收和散射的光学深度为τa>>1，τ>>1，并且τ>>τa，则随机游走场景中的有效吸收光学深度将等于\surd τa（τa+τ）。然而，我们表明，对于频率为ν0的共振散射，有效吸收光学深度将甚至大于τ（ν0）。如果尘埃散射和吸收的截面与频率无关，则共振散射给出的频率剖面的双峰结构基本上与尘埃无关。也就是说，灰尘既不会导致双峰轮廓的宽度变窄也不会导致双峰轮廓的宽度变宽。另一个结果是，光学厚晕中Lyα光子传输的时间尺度也基本上与尘埃散射无关，即使散射是各向异性的。这是因为这些时间尺度主要由频率空间中的传输确定，而当散射的截面与波长无关时，各向同性或各向异性的灰尘散射不会影响频率空间中的传输行为。这一结果不支持尘埃将导致光学厚晕Lyα光子源亮度分布平滑化的推测。
英文摘要：
We investigate the effects of dust on Lyα photons emergent from an optically thick medium by solving the integro-differential equation of the radiative transfer of resonant photons. To solve the differential equations numerically we use the Weighted Essentially Non-oscillatory method (WENO). Although the effects of dust on radia-tive transfer is well known, the resonant scattering of Lyα photons makes the problem non-trivial. For instance, if the medium has the optical depth of dust absorption and scattering to be τa>>1, τ>>1, and τ>>τa, the effective absorption optical depth in a random walk scenario would be equal to \surd τa(τa+τ). We show, however, that for a resonant scattering at frequency ν0, the effective absorption optical depth would be even larger than τ(ν0). If the cross section of dust scattering and absorption is frequency-independent, the double-peaked structure of the frequency profile given by the resonant scattering is basically dust-independent. That is, dust causes neither narrowing nor widening of the width of the double peaked profile. One more result is that the time scales of the Lyα photon transfer in the optically thick halo are also basically independent of the dust scattering, even when the scattering is anisotropic. This is because those time scales are mainly determined by the transfer in the frequency space, while dust scattering, either isotropic or anisotropic, does not affect the behavior of the transfer in the frequency space when the cross section of scattering is wavelength-independent. This result does not support the speculation that dust will lead to the smoothing of the brightness distribution of Lyα photon source with optical thick halo.
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