Modelling the dust emission from dense interstellar clouds: disentangling the effects of radiative transfer and dust properties
Modelling the dust emission from dense interstellar clouds: disentangling the effects of radiative transfer and dust properties
复制标题
DOI：
10.1051/0004-6361/201118420
复制
发表时间：
2012-02
影响因子：
6.5
通讯作者：
N. Ysard;M. Juvela;K. Demyk;V. Guillet;A. Abergel;J. Bernard;J. Malinen;C. M'eny;L. Montier;D. Paradis;I. Ristorcelli;L. Verstraete
中科院分区：
文献类型：
--
作者：
N. Ysard;M. Juvela;K. Demyk;V. Guillet;A. Abergel;J. Bernard;J. Malinen;C. M'eny;L. Montier;D. Paradis;I. Ristorcelli;L. Verstraete
语境。尘埃排放越来越多地用作星际介质中质量的示踪剂。通过普朗克和赫歇尔天文台的结合，我们现在拥有观察密集和冷分子云所需的光谱覆盖范围和角分辨率。然而，由于这些云在短波长下光学上很厚，但在长波长下光学上很薄，因此如果不对辐射传输进行适当的处理，就很难得出有关尘埃特性的任何结论。目标。我们的目标是理清辐射传输和灰尘特性对长波长灰尘发射变化的影响。这使我们能够为观察者提供工具来分析浓密云层产生的粉尘排放。方法。我们对视觉消光在 1 到 20 星等之间的圆柱形云进行建模，在标准星际辐射场的照射下，并使用蒙特卡罗代码进行完整的辐射传输计算。使用 DustEM 代码求解代表高银河纬度 (DHGL) 尘埃的无定形碳和硅酸盐、涂有碳幔的碳和硅酸盐颗粒以及碳和硅酸盐的混合聚集体的 DustEM 代码。我们还允许颗粒的光学特性随波长和温度的变化。我们通过使用标准 χ 2 拟合方法将灰尘发射与改进的黑体拟合来确定观测到的色温 T color 和发射率光谱指数 β color ，以便将我们的模型与观测结果进行比较。结果。辐射传输效应既不能解释低 T 颜色、在稠密云中心测量到的亚毫米发射率增加，也不能解释所考虑模型中观察到的 β 颜色 - T 颜色反相关性。在建模数据中添加真实的噪声，我们表明这不太可能是在无星云中观察到的 β 颜色 - T 颜色反相关的唯一解释，这可能可以通过颗粒光学特性随温度的内在变化来解释。类似地，较高的亚毫米发射率和较低的T颜色必须源于颗粒光学特性的变化，这可能是由于它们生长形成多孔聚集体引起的。我们发现，由于 λ ≲ 300 μm 的辐射传输效应，以及不同颗粒群在较长波长下的混合，我们很难从颗粒发射的光谱变化来推断颗粒的性质。最后，由于 T 颜色与云最内层的“真实”尘埃温度之间的差异，当使用改进的黑体拟合确定时，柱密度被低估。
Context. Dust emission is increasingly used as a tracer of the mass in the interstellar medium. With the combination of Planck and Herschel observatories, we now have both the spectral coverage and the angular resolution required to observe dense and cold molecular clouds. However, as these clouds are optically thick at short wavelengths but optically thin at long wavelengths, it is tricky to conclude anything about dust properties without a proper treatment of the radiative transfer. Aims. Our aim is to disentangle the effects of radiative transfer and dust properties on the variations in the dust emission at long wavelengths. This enables us to provide observers with tools to analyse the dust emission arising from dense clouds. Methods. We model cylindrical clouds with visual extinctions between 1 and 20 mag, illuminated by the standard interstellar radiation field, and carry out full radiative transfer calculations using a Monte Carlo code. Dust temperatures are solved using the DustEM code for amorphous carbons and silicates representative of dust at high Galactic latitude (DHGL), carbon and silicate grains coated with carbon mantles, and mixed aggregates of carbon and silicate. We also allow for variations in the optical properties of the grains with wavelength and temperature. We determine observed colour temperatures, T colour , and emissivity spectral indices, β colour , by fitting the dust emission with modified blackbodies using a standard χ 2 fitting method, in order to compare our models with observational results. Results. Radiative transfer effects can explain neither the low T colour , the increased submillimetre emissivity measured at the centre of dense clouds, nor the observed β colour − T colour anti-correlation for the models considered. Adding realistic noise to the modelled data, we show that it is unlikely to be the only explanation of the β colour − T colour anti-correlation observed in starless clouds, which may instead be explained by intrinsic variations in the grain optical properties with temperature. Similarly the higher submillimetre emissivity and the low T colour have to originate in variations in the grain optical properties, probably caused by their growth to form porous aggregates. We find it difficult to infer the nature of the grains from the spectral variations in their emission, owing to radiative transfer effects for λ ≲ 300 μ m, and to the mixture of different grain populations for longer wavelengths. Finally, the column density is underestimated when determined with modified blackbody fitting because of the discrepancy between T colour and the “true” dust temperature in the innermost layers of the clouds.