Context. Carbonaceous nano-grains play a fundamental role in the physico-chemistry of the interstellar medium (ISM) and especially of photon-dominated regions (PDRs). Their properties vary with the local physical conditions and affect the local chemistry and dynamics. Aims. We aim to highlight the evolution of carbonaceous nano-grains in three different PDRs and propose a scenario of dust evolution as a response to the physical conditions. Methods. We used Spitzer/IRAC (3.6, 4.5, 5.8, and 8 $μ$m) and Spitzer/MIPS (24 $μ$m) together with Herschel/PACS (70 $μ$m) to map dust emission in IC63 and the Orion Bar. To assess the dust properties, we modelled the dust emission in these regions using the radiative transfer code SOC together with the THEMIS dust model. Results. Regardless of the PDR, we find that nano-grains are depleted and that their minimum size is larger than in the diffuse ISM (DISM), which suggests that the mechanisms that lead nano-grains to be photo-destroyed are very efficient below a given critical size limit. The evolution of the nano-grain dust-to-gas mass ratio with both G0 and the effective temperature of the illuminating star indicates a competition between the nano-grain formation through the fragmentation of larger grains and nano-grain photo-destruction. We modelled dust collisions driven by radiative pressure with a classical 1D approach to show that this is a viable scenario for explaining nano-grain formation through fragmentation and, thus, the variations observed in nano-grain dust-to-gas mass ratios from one PDR to another. Conclusions. We find a broad variation in the nano-grain dust properties from one PDR to another, along with a general trend of nano-grain depletion in these regions. We propose a viable scenario of nano-grain formation through fragmentation of large grains due to radiative pressure-induced collisions.
Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM III. Elemental depletion and shortcomings of the current physico-chemical models
本文研究了星际介质中的元素损耗。我们结合了银河系模型的结果，描述了致密核形成过程中的气体物理条件与全气体颗粒化学模型。在扩散和稠密介质之间的过渡期间，最初在气体中的原子元素的储存器逐渐在尘埃颗粒上耗尽（对于那些离子来说，有一个中和阶段）。当密度大于100 cm $^{-3}$时，该过程变得有效。如果致密材料回到扩散状态，这些元素又回到气相，因为冰上分子的光离解，然后从颗粒上热解吸。对于密度低于10 cm $^{-3}$的颗粒或气相中的分子形式，颗粒上没有残留物。一个例外是氯，它在低密度下有效地转化。我们现有的气体颗粒化学模型不能再现在扩散介质中观察到的原子的耗尽，除了Cl，其气体丰度遵循在密度小于10 cm $^{-3}$的介质中观察到的气体丰度。这表明关键的过程（可能涉及化学吸附和/或冰辐射深刻改变冰的性质）缺失。
Interstellar Silicon Depletion and the Ultraviolet Extinction
Spinning small silicate grains were recently invoked to account for the Galactic foreground anomalous microwave emission. These grains, if present, will absorb starlight in the far ultraviolet (UV). There is also renewed interest in attributing the enigmatic 2175 Angstrom interstellar extinction bump to small silicates. To probe the role of silicon in the UV extinction, we explore the relations between the amount of silicon required to be locked up in silicates [Si/H]_{dust} and the 2175 Angstrom bump or the far-UV extinction rise, based on an analysis of the extinction curves along 46 Galacti…
Observing Co-Located Neutral and Ionized Gas-Phase Iron Depletion in the Magellanic Clouds
University of Washington（华盛顿大学）；Indiana University（印第安纳大学）；Space Telescope Science Institute（太空望远镜科学研究所）；University of California Santa Cruz（加州大学圣克鲁兹分校）
METAL: The Metal Evolution, Transport, and Abundance in the Large Magellanic Cloud Hubble program. I. Overview and Initial Results
大麦哲伦星云中的金属演化、运输和消耗（Metal Evolution，Transport，and Abdeletion in the LMC，简称METAL）是哈勃太空望远镜上的一个大型第24周期计划，旨在测量大麦云（LMC）中半太阳金属丰度的尘埃消光特性和星际消耗。101轨道计划包括对33颗1150 A和3180 A之间的LMC大质量恒星的COS和STIS光谱，以及7个NUV-NIR滤波器的并行WFC 3成像。从光谱获得的气相中的硅的分数（耗尽）随着氢柱密度的增加而减少。硅的耗尽模式在银河系、大麦哲伦星云和SMC之间不同，硅的耗尽水平几乎完全抵消了金属丰度的差异，导致在给定的氢柱密度下，这些星系中的气相丰度恒定。硅耗尽与绝对选择性消光R $_V$呈线性相关，表明气体耗尽与尘埃粒度之间存在联系。消光图来自于并行成像中的恒星测光，可以与赫歇尔和斯皮策的FIR图像进行比较，以估计LMC金属丰度下尘埃的发射率。完整的METAL样品的消耗，紫外消光曲线和消光图将告知LMC中尘埃颗粒的丰度，大小，成分和光学性质，全面提高我们对尘埃性质的理解，以及估计附近和高红移星系中尘埃气体质量，恒星形成率和历史的准确性。这篇综述性论文描述了METAL调查的目标、设计、数据简化和初步结果。
Chemical modelling of dust-gas chemistry within AGB outflows -- II. Effect of the dust-grain size distribution
AGB星和超新星是构成星际介质的主要物质。AGB星形成的尘埃颗粒被认为很大。然而，由于尘埃在其流出物中的成核和生长仍然不清楚，尘埃粒度分布（GSD）是未知的。这是一个重要的不确定性，我们的知识的化学和物理历史的星际尘埃，作为AGB尘埃形式的$\sim$ 70%的起点，其演变。我们扩展了我们的化学动力学模型，其中包括一个全面的粉尘-气体化学。现在允许GSD偏离通常假设的标准Mathis et al.（1977）分布。我们发现，特定的GSD可以显着影响内流出的粉尘-气体化学。我们的研究结果表明，耗尽的气相物种的水平取决于平均晶粒表面积的GSD。当使用一系列跃迁时，可以从分子发射线的观测中检索气相丰度分布及其可能的耗尽。由于GSD处方内的简并性，无法检索特定参数，仅检索平均晶粒表面积（下限）。尽管如此，这可以区分主要由大颗粒或小颗粒组成的灰尘。我们表明，当结合其他可观测的，如光谱能量分布和偏振光，耗尽水平从分子气相丰度分布可以限制难以捉摸的GSD的尘埃AGB流出到ISM。
