Effect of dust grain porosity on the appearance of protoplanetary disks
Effect of dust grain porosity on the appearance of protoplanetary disks
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DOI：
10.1051/0004-6361/201323176
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发表时间：
2014-07
期刊：
影响因子：
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通讯作者：
F. Kirchschlager;S. Wolf
中科院分区：
文献类型：
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作者：
F. Kirchschlager;S. Wolf
我们从理论上分析由多孔尘埃颗粒组成的原行星盘。在对原行星盘观测的分析中，通常假定尘埃相由球形颗粒组成，从而允许应用米氏散射形式。然而，在现实中，预计尘埃颗粒的形状与球体的形状有很大的偏差。我们研究了孔隙率高达60%的尘埃颗粒对原行星盘温度分布和可观测外观的影响。我们利用辐射传输模型模拟了温度分布、光谱能量分布以及空间分辨的强度和极化图。采用离散偶极子近似方法计算了多孔颗粒的光学性质。我们发现，在光学波长范围内，多孔颗粒的通量高于致密的球形颗粒。在9.7 um处的硅酸盐峰的剖面强烈依赖于颗粒孔隙度。温度分布在垂直于背板方向上有明显的变化。此外，模拟极化图显示，当考虑多孔颗粒时，极化度增加了约4倍，而不考虑圆盘倾角。偏振方向在选择的圆盘区域是相反的，这取决于波长、颗粒孔隙度和圆盘倾角。我们讨论了这种效应的几种可能的解释，并发现多次散射最能解释这种效应。孔隙度影响原行星盘的可观测外观。特别是，极化反转表现出与晶粒孔隙度的依赖关系。孔隙率改变了盘片内的物理条件，影响了晶粒的生长和盘片的演化过程。
We theoretically analyze protoplanetary disks consisting of porous dust grains. In the analysis of observations of protoplanetary disks the dust phase is often assumed to consist of spherical grains, allowing one to apply the Mie scattering formalism. However, in reality, the shape of dust grains is expected to deviate strongly from that of a sphere. We investigate the influence of porous dust grains on the temperature distribution and observable appearance of protoplanetary disks for dust grain porosities of up to 60 %. We performed radiative transfer modeling to simulate the temperature distribution, spectral energy distribution, and spatially resolved intensity and polarization maps. The optical properties of porous grains were calculated using the method of discrete dipole approximation. We find that the flux in the optical wavelength range is for porous grains higher than for compact, spherical grains. The profile of the silicate peak at 9.7 um strongly depends on the degree of grain porosity. The temperature distribution shows significant changes in the direction perpendicular to the midplane. Moreover, simulated polarization maps reveal an increase of the polarization degree by a factor of about four when porous grains are considered, regardless of the disk inclination. The polarization direction is reversed in selected disk regions, depending on the wavelength, grain porosity, and disk inclination. We discuss several possible explanations of this effect and find that multiple scattering explains the effect best. Porosity influences the observable appearance of protoplanetary disks. In particular, the polarization reversal shows a dependence on grain porosity. The physical conditions within the disk are altered by porosity, which might have an effect on the processes of grain growth and disk evolution.