Optical Properties of Interstellar Dust

项目摘要：星际尘埃的光学性质在星际介质的热、化学和流体动力学以及行星和恒星的形成中起着重要的作用。尘埃会衰减星系的光学和紫外线(UV)光谱，并从红外到微波波段发射。更好地理解星际尘埃对天体物理学有着广泛的影响。我们对星际尘埃的了解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。为了解释观测和检验尘埃模型，需要计算模型颗粒的散射和吸收特性。光的散射和吸收也会产生重要的动力学后果，因为这会对尘埃颗粒施加力和力矩。在这项工作中，将对星际尘埃的光学性质进行一些相关的研究。研究人员，德雷恩教授和一名研究生，计划开发计算小颗粒或纳米结构的散射和吸收的改进方法。他们计划进一步开发和应用离散偶极近似(DDA)作为一种计算不规则颗粒和复合颗粒在从紫外线到红外的波长下的散射和吸收的技术。将寻求一种新的方法来确定表面校正？偶极子极化率，以提高DDA的精度。离散偶极子近似程序DDSCAT的新版本也将实现其他算法改进，该程序是一个公开可用的DDA程序，用于计算一般目标形状的光散射和吸收。反常衍射理论将用于X射线能量。最初的目标是创建一个图书馆？对于选定的颗粒大小、形状和成分，在广泛的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？由标准的弹道聚集产生的星团？程序，以及产生更紧凑、更不脆弱、可能更现实的集群的其他聚合规则。它还计划研究不规则但致密的颗粒的散射和吸收，使用变种的高斯球？生成随机形状的技术。这一散射结果库在随后的光散射和吸收研究中将是有用的，应该具有广泛的适用性。该图书馆计划在首席研究员S的网站上公开可用。作为这项工作的一部分，计划为扩散ISM中的硅酸盐材料开发一种改进的介电功能。介电函数需要重现观测到的星际硅酸盐在消光和极化方面的特征，并与其他天体物理限制一致，包括远红外和亚毫米辐射以及星际丰度。硅酸盐消光与极化之间的关系对颗粒形状和介电函数很敏感。我们的目标是成功地复制这两者，这将使人们对所产生的介电函数和颗粒形状有一定的信心。在创建了各种类型颗粒的散射属性库后，将构建星际尘埃的模型，并调整其粒度分布，以重现与波长相关的消光、星光的波长相关偏振、红外发射和X射线散射特性的观测。计划用不同的谷物类型来实现这一点，包括致密谷物和蓬松谷物。这将有助于缩小给出与观测相一致的模型结果的颗粒(例如，绒毛颗粒)的种类。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。拟议的工作将产生广泛的跨学科影响。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。我们计算粒子和结构的吸收和散射的能力的进一步改进将超越天体物理学的价值。PI将继续支持DDSCAT包，并通过WWW将其公开提供。

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.