Scattering-produced (sub)millimetre polarization in inclined discs: optical depth effects, near–far side asymmetry and dust settling
Scattering-produced (sub)millimetre polarization in inclined discs: optical depth effects, near–far side asymmetry and dust settling
复制标题
倾斜圆盘中散射产生的（亚）毫米偏振：光学深度效应、近远端不对称性和灰尘沉降
DOI：
10.1093/mnras/stx1951
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发表时间：
2017
影响因子：
4.8
通讯作者：
Stephens, Ian W.
中科院分区：
文献类型：
--
作者：
Yang, Haifeng;Li, Zhi-Yun;Looney, Leslie W.;Girart, Josep M.;Stephens, Ian W.
在（亚）毫米波长的盘极化是革命性的由ALMA观测，但其起源仍然不确定。尘埃散射最近被认为是极化的一个潜在因素，尽管它的基本性质尚未被彻底探索。在这里，我们量化了光学深度对倾斜圆盘散射诱导偏振的影响，通过分析说明，近似半解析建模使用辐射传递方程的形式解，和蒙特卡罗模拟相结合。我们发现，如果圆盘具有光学厚度且散射颗粒尚未沉降到中间平面，则圆盘近侧的偏振强度明显高于远侧。这种不对称是一种简单的几何效应的结果：圆盘表面的近面比远面更接近边缘。这是一个强大的特征，可用于区分散射诱导极化与其他机制，如排列的晶粒。对于几何上较薄的尘埃盘来说，这种不对称性较弱。因此，它为尘埃沉降打开了一个令人兴奋的新窗口。我们从边缘盘的尘埃连续成像中发现轶事证据，在最年轻的（0类）盘中，大颗粒尚未沉淀，但在较老的盘中则越来越多。倾斜圆盘的偏振数据证实了这一趋势，表明较年轻的圆盘具有更明显的近远侧不对称性，因此颗粒沉降较少。如果得到证实，这一趋势将对颗粒演化产生深远影响，并最终对星子和行星的形成产生深远影响。
Disc polarization at (sub)millimetre wavelengths is being revolutionized by ALMA observationally, but its origin remains uncertain. Dust scattering was recently recognized as a potential contributor to polarization, although its basic properties have yet to be thoroughly explored. Here, we quantify the effects of optical depth on the scattering-induced polarization in inclined discs through a combination of analytical illustration, approximate semi-analytical modelling using formal solution to the radiative transfer equation, and Monte Carlo simulations. We find that the near-side of the disc is significantly brighter in polarized intensity than the far-side, provided that the disc is optically thickandthat the scattering grains have yet to settle to the mid-plane. This asymmetry is the consequence of a simple geometric effect: the near-side of the disc surface is viewed more edge-on than the far-side. It is a robust signature that may be used to distinguish the scattering-induced polarization from that by other mechanisms, such as aligned grains. The asymmetry is weaker for a geometrically thinner dust disc. As such, it opens an exciting new window on dust settling. We find anecdotal evidence from dust continuum imaging of edge-on discs that large grains are not yet settled in the youngest (Class 0) discs, but become more so in older discs. This trend is corroborated by the polarization data in inclined discs showing that younger discs have more pronounced near–far side asymmetry and thus less grain settling. If confirmed, the trend would have far-reaching implications for grain evolution and, ultimately, the formation of planetesimals and planets.
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