Interstellar Dust: Optics and Models

这个研究项目的学术价值是广泛的。尘埃减少了来自星系的光，发出自己的光，在星系中恒星系统之间物质的热、化学和流体动力学中，以及在行星和恒星的形成中，扮演着重要的角色。更好地理解星际尘埃颗粒的物理性质将对从恒星和行星的形成到活动星系核附近物质的各种天体物理现象的研究具有重要意义。我们对星际尘埃的知识主要来自对尘埃颗粒对电磁辐射的吸收、散射和发射的远程观测。尘埃颗粒的散射和吸收特性是解释观测结果和检验尘埃模型所必需的。此外，光的散射和吸收对尘埃颗粒施加力和力矩，具有重要的动力学后果。布鲁斯·德雷恩博士和他的研究团队将开展一系列关于星际尘埃光学性质的研究，并开发用于计算不规则几何小粒子光学性质的改进工具。这些工具将被用来寻找新的最先进的星际尘埃模型(成分、形状和尺寸分布)，这些模型与观测约束一致。最初的目标是创建一个计算横截面的“库”，用于在广泛的波长范围内对选定的颗粒大小、形状和组成进行散射和吸收。颗粒的几何形状将包括球体，由球体随机凝聚而成的蓬松颗粒，以及不规则但致密的颗粒。将使用最先进的代码。离散偶极子近似(DDA)将用于不规则颗粒几何形状。我们将使用一种将反常绕射理论应用于一般几何形状的新程序，对X射线能量下的吸收和微分散射截面进行精确计算。这些代码将被记录下来，并在WWW上公开提供(开源)。Draine博士将继续开发和支持使用DDA进行计算的开源代码DDSCAT。横断面库将在万维网上提供。对于每一种谷物，研究小组将计算各种加热辐射场的温度分布函数；温度分布函数库也将在WWW上提供。这些将是有价值的社区资源。计算的截面和温度分布函数将被用来构建迄今为止最好的星际尘埃模型，其尺寸分布将被调整，以重现与波长相关的消光、星光的偏振、偏振红外发射和X射线散射的观测。模型将建造不同的颗粒形状，包括致密的颗粒和蓬松的颗粒。星际颗粒可能包括磁性物质。该团队将继续研究这种颗粒发射的磁偶极辐射的强度和极化。这种磁偶极子辐射在毫米和厘米波长可能是重要的，为“星系前景”贡献了一个新的极化成分，影响了宇宙微波背景的极化研究。计算纳米结构的散射和吸收的改进方法具有广泛的适用性，超越了天体物理学。小颗粒的光学在许多科学领域都很重要，包括大气科学、海洋学、行星科学、燃烧科学、海洋生物学和纳米颗粒研究。DDSCAT已被用户应用于所有这些领域。DDSCAT包将继续得到Dr.Draine的支持，并通过WWW公开提供。该研究项目包括研究生部分，并计划参加本科生课程。因此，拟议的研究将有助于培养未来的科学家。

The intellectual merit of this research project is broad. Dust reduces the light from galaxies, sends out its own light, and plays an important role in the thermo-, chemo-, and fluid-dynamics of the matter between the star systems in a galaxy, and in the formation of planets and stars. An improved understanding of the physical properties of interstellar dust grains will have implications for studies of diverse astrophysical phenomena, from formation of stars and planets to the material near active galactic nuclei.Our knowledge of interstellar dust derives primarily from remote observations of absorption, scattering, and emission of electromagnetic radiation by dust grains. The scattering and absorption properties of dust grains are needed to interpret observations, and to test dust models. In addition, scattering and absorption of light exerts forces and torques on dust grains, with important dynamical consequences. Dr. Bruce Draine and his research team will carry out a number of investigations concerning the optical properties of interstellar dust, and development of improved tools for calculating the optical properties of small particles with irregular geometries. These tools will be used to find new state-of-the-art models (composition, shape, and size distribution) for interstellar dust that are consistent with observational constraints.The initial objective is to create a "library" of calculated cross sections for scattering and absorption over a broad range of wavelengths, for selected grain sizes, shapes, and compositions. Grain geometries will include spheroids, fluffy grains built up from random coagulation of spheres, and grains that are irregular but compact. State-of-the-art codes will be employed. The discrete dipole approximation (DDA) will be used for irregular grain geometries. Accurate calculations of absorption and differential scattering cross sections at X-ray energies will be carried out using a new code applying anomalous diffraction theory to general geometries. This code will be documented and made publicly-available (open source) on the WWW. Dr. Draine will continue to develop and support the open-source code DDSCAT for calculations using the DDA. The cross-section library will be made available on the WWW. For each grain, the team will calculate the temperature distribution function for various heating radiation fields; the library of temperature distribution functions will also be made available on the WWW. These will be valuable community resources.The calculated cross-sections and temperature distribution functions will be used to construct the best-to-date interstellar dust models with size distributions adjusted to reproduce observations of wavelength-dependent extinction, wavelength-dependent polarization of starlight, polarized infrared emission, and X-ray scattering. Models will be built with different grain shapes, including compact grains and fluffy grains. Interstellar grains may include magnetic material. The team will continue study of the intensity and polarization of the magnetic dipole radiation emitted by such grains. This magnetic dipole radiation may be important at mm and cm wavelengths, contributing a new polarized component to the "galactic foreground", affecting studies of the polarization of the cosmic microwave background.This work has broad interdisciplinary impact. Improved methods for calculating scattering and absorption by nanostructures have wide applicability, beyond astrophysics. The optics of small particles is important in many scientific fields, including atmospheric science, oceanography, planetary science, combustion science, marine biology, and nanoparticle studies. DDSCAT has already been applied by users in all of these areas. The DDSCAT package will continue to be supported by Dr. Draine and made publicly available via the WWW. The research program includes a graduate student component, and undergraduate participation is planned. The proposed research will thereby contribute to training future scientists.