论文标题
马头星云的尘埃演变
Dust evolution across the Horsehead Nebula
论文作者
论文摘要
星际灰尘表面上的微物理过程与灰尘特性（即灰尘组成，大小和形状）紧密连接，并在星际介质（ISM）的许多现象中起关键作用。 ISM中物理条件（即密度，气温）的巨大差异触发了灰尘特性的演变。对灰尘如何在物理条件下演变的分析是对星际尘埃的更彻底理解的垫脚石。本文的目的是突出马头星云PDR地区的尘埃演化。我们将Spitzer/IRAC（3.6、4.5、5.8和8μm）与Herschel/PAC（70和160μm）以及Herschel/Spire（250、350和500μm）一起使用Spitzer/MIPS（24μm），以绘制在整个发射范围内尘埃的空间分布。我们使用Themis星际灰尘模型以及3D辐射传输代码SOC对灰尘发射和散射进行建模。我们发现马头辐照外部的纳米粒尘埃气（灰尘与气体）比弥漫性ISM低6至10倍。它们的最小尺寸是扩散ISM和大小分布的幂律指数的2至2.25倍，比弥漫性ISM低1.1至1.4倍。关于马头的较密集的部分，有必要使用进化的谷物（即有或没有冰地幔的聚集体）。不可能使用漫射介质中的晶粒来解释观测值。因此，我们提出以下方案来解释我们的结果。在马头的外部，所有纳米粒尚未通过聚集体的照片碎片和对辐射场敏感的最小的纳米晶粒的照片碎片进行拍摄。在马头的内部，谷物最有可能由多组合的，底漆的聚集体组成。
Micro-physical processes on interstellar dust surfaces are tightly connected to dust properties (i.e. dust composition, size and shape) and play a key role in numerous phenomena in the interstellar medium (ISM). The large disparity in physical conditions (i.e. density, gas temperature) in the ISM triggers an evolution of dust properties. The analysis of how dust evolves with the physical conditions is a stepping-stone towards a more thorough understanding of interstellar dust. The aim of this paper is to highlight dust evolution in the Horsehead Nebula PDR region. We use Spitzer/IRAC (3.6, 4.5, 5.8 and 8 μm), Spitzer/MIPS (24 μm) together with Herschel/PACS (70 and 160 μm) and Herschel/SPIRE (250, 350 and 500 μm) to map the spatial distribution of dust in the Horsehead over the entire emission spectral range. We model dust emission and scattering using the THEMIS interstellar dust model together with the 3D radiative transfer code SOC. We find that the nano-grains dust-to-gas ratio in the irradiated outer part of the Horsehead is 6 to 10 times lower than in the diffuse ISM. Their minimum size is 2 to 2.25 times larger than in the diffuse ISM and the power-law exponent of their size distribution, 1.1 to 1.4 times lower than in the diffuse ISM. Regarding the denser part of the Horsehead, it is necessary to use evolved grains (i.e. aggregates, with or without an ice mantle). It is not possible to explain the observations using grains from the diffuse medium. We therefore propose the following scenario to explain our results. In the outer part of the Horsehead, all the nano-grains have not yet had time to re-form completely through photo-fragmentation of aggregates and the smallest of the nano-grains that are sensitive to the radiation field are photo-destroyed. In the inner part of the Horsehead, grains most likely consist of multi-compositional, mantled aggregates.
