Optical Properties of Interstellar Dust

项目摘要：星际尘埃的光学性质尘埃在星际介质的热动力学、化学动力学和流体动力学以及行星和恒星的形成中发挥着重要作用。尘埃衰减了星系的光学和紫外光谱，并发射出从红外到微波的波长。更好地了解星际尘埃对天体物理学有着广泛的影响。我们对星际尘埃的认识主要来自于对尘埃颗粒吸收、散射和发射电磁辐射的远距离观测。为了解释观测结果和测试沙尘模型，需要计算模型颗粒的散射和吸收特性。光的散射和吸收也有重要的动力学后果，因为这会对尘埃颗粒施加力和力矩。在这项工作中，将进行一些有关星际尘埃光学特性的相关研究。研究人员Draine教授和一名研究生计划开发改进的方法来计算小颗粒或纳米结构的散射和吸收。他们计划进一步开发和应用离散偶极近似（DDA）作为计算不规则颗粒和复合颗粒在紫外到红外波长处的散射和吸收的技术。将采用新的方法来确定？表面校正？偶极极化率，以提高DDA的准确性。其他算法的改进也将被实施，并包括在一个新版本的离散偶极子近似代码DDSCAT，这是一个公开的DDA代码计算光散射和吸收一般目标形状。异常衍射理论将用于X射线能量。最初的目标是建立一个？图书馆？在选定的粒度、形状和成分的宽波长范围内的散射和吸收特性。颗粒几何形状将包括由较小单体组成的颗粒，包括？毛茸茸的？标准创建的集群？弹道聚合程序，以及其他聚合规则，导致集群更紧凑，更不脆弱，可能更现实。它还计划研究散射和吸收的颗粒是不规则的，但紧凑的，使用的变体？高斯球？技术来生成随机形状。这个库的散射结果将是有用的光散射和吸收的后续研究，应该有广泛的适用性。是否计划在主要研究者上公开图书馆？的网站。作为这项工作的一部分，计划在扩散ISM的硅酸盐材料的改进的介电函数的发展。介电函数需要再现所观察到的星际硅酸盐的消光和偏振功能，并与其他天体物理约束，包括远红外和亚毫米发射和星际丰度一致。硅酸盐消光和极化之间的关系是敏感的晶粒形状和介电函数。目标是成功地再现这两种情况，这将给最终的介电函数和颗粒形状带来一定的信心。在创建了各种类型颗粒的散射特性库之后，将构建星际尘埃模型，并调整其尺寸分布，以再现波长相关消光，星光波长相关偏振，红外发射和X射线散射特性的观测结果。计划用不同的谷物类型来做这件事，包括紧凑的谷物和蓬松的谷物。这将有助于缩小谷物的种类（例如，蓬松的颗粒），给出与观测相符的模型结果。因此，对不同情况的研究将表明哪种颗粒类型将被排除为主要的星际颗粒成分。这项拟议中的工作将产生广泛的跨学科影响。小颗粒的光学特性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。进一步提高我们计算粒子和结构的吸收和散射的能力，其价值将超出天体物理学。PI将继续支持DDSCAT包，并通过万维网公开提供。

Project Summary: Optical Properties of Interstellar DustDust plays an important role in the thermo-, chemo-, and hydrodynamics of the interstellar medium and in the formation of planets and stars. Dust attenuates the optical and ultraviolet (UV) spectra of galaxies, and it emits from the infrared to microwave wavelengths. A better understanding of interstellar dust has broad impact across astrophysics. Our knowledge of interstellar dust is derived primarily from remote observation of absorption, scattering, and emission of electromagnetic radiation by dust grains. In order to interpret observations and test dust models, the scattering and absorption properties of model grains need to be calculated. There are also important dynamical consequences from scattering and absorption of light because this exerts forces and torques on dust grains.In this work a number of related investigations concerning the optical properties of interstellar dust will be carried out. The investigators, Professor Draine and a graduate student, plan to develop improved methods for calculating scattering and absorption by small particles or nanostructures. They plan further develop and apply the discrete dipole approximation (DDA) as a technique for calculating scattering and absorption by irregular grains and composite grains at wavelengths from the ultraviolet to the infrared. A new approach will be pursued to determine ?surface corrected? dipole polarizabilities to improve the accuracy of the DDA. Other algorithmic improvements will also be implemented and included in a new release of the discrete dipole approximation code DDSCAT, which is a publicly-available DDA code for calculating light scattering and absorption by general target shapes.Anomalous diffraction theory will be used at X-ray energies. The initial objective will be to create a ?library? of scattering and absorption properties over a broad range of wavelengths for selected grain sizes, shapes, and compositions. Grain geometries will include grains that are built up from smaller monomers, including the ?fluffy? clusters created by the standard ?ballistic aggregation? procedure, as well as by other aggregation rules that result in clusters that are more compact, less fragile, and possibly more realistic. It is also planned to study the scattering and absorption by grains that are irregular but compact, using a variant of the ?Gaussian spheres? technique to generate random shapes. This library of scattering results will be useful in subsequent studies of light scattering and absorption and should have wide applicability. It is planned to make the library publicly available on the principal investigator?s web site. The development of an improved dielectric function for the silicate material in the diffuse ISM is planned as part of this work. The dielectric function is required to reproduce the observed interstellar silicate features in both extinction and polarization, and to be consistent with other astrophysical constraints including far-infrared and submm emission and interstellar abundances. The relationship between the silicate extinction and polarization is sensitive to grain shape and to the dielectric function. The goal is to successfully reproduce both, which will give some confidence in the resulting dielectric function and grain shape.After creating the library of scattering properties for grains of various types, models for interstellar dust will be constructed with size distributions adjusted to reproduce observations of wavelength-dependent extinction, wavelength-dependent polarization of starlight, infrared emission, and X-ray scattering properties. It is planned to do this with different grain types, including compact grains and fluffy grains. This will help to narrow down the kinds of grains (e.g., fluffy grains) that give model results compatible with observations. A study of the different cases thus will show which grain type would be ruled out as a major interstellar grain component.The proposed work will have broad interdisciplinary impact. The optics of small particles is important in many scientific fields, including atmospheric science, oceanology, planetary science, combustion science, marine biology, and nanoparticle studies. DDSCAT has already been applied by users in all of these areas. Further improvements in our ability to calculate absorption and scattering by particles and structures will be of value beyond astrophysics. The PI will continue to support the DDSCAT package and to make it publicly-available via the WWW.