Optics and Dynamics of Interstellar Dust

我们对星际尘埃的认识主要来源于尘埃颗粒对电磁辐射的吸收、散射和发射。为了检验一个尘埃模型，或者解释观测结果，我们必须能够计算模型颗粒的散射和吸收特性。此外，光的散射和吸收对尘埃颗粒施加力和扭矩，具有重要的动力学后果。Bruce Draine博士将指导一个项目，进一步发展离散偶极近似，作为一种计算从紫外线到红外线波长的颗粒散射和吸收的技术。将努力改进基本近似，并实施算法改进以加快所需的数值计算。Draine博士将把这些改进包括在DDSCAT的新版本中，这是一个可公开获得的软件包，用于计算一般目标形状的电磁散射和吸收。将开发一个单独的代码，使用反常衍射理论来计算任意形状的颗粒对x射线的散射和吸收。此代码也将公开提供。这些代码将应用于各种谷物几何形状，包括蓬松的聚集体和不规则的紧凑形状，以计算从红外到x射线波长的吸收和散射。目的是调查这种形状可能代表星际尘埃的可能性。改进的颗粒模型将基于观测约束，包括红外发射和波长相关消光。尘埃的微波辐射已经被探测到，对光谱的观测知识也在不断增加。这种辐射的强度和光谱似乎与快速旋转的非常小的尘埃颗粒的旋转辐射一致。Draine博士将研究非常小的颗粒的旋转动力学和电偶极子发射，目的是建立与最近观测结果更一致的模型，包括观测到的微波发射的区域变化。人们在半个多世纪前就已经知道了排列整齐的尘埃颗粒对星光的偏振作用，但还没有得到令人满意的解释。众所周知，星光施加的力矩在晶粒动力学中起着重要作用。这项工作将包括对受星光扭矩和内部热波动影响的不规则颗粒的旋转动力学的广泛研究。目标是确定，在我们现在所理解的颗粒动力学下，一群被各向异性星光照射的不规则尘埃颗粒是否会部分对准，其对准程度与颗粒大小与偏振波长依赖的观测结果一致。如果颗粒模型确实发展出与观测相符的排列，那么星际颗粒排列之谜将最终得到解决。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起着重要的作用，它使星系的光学光谱和紫外光谱衰减，并从微波到i波段发射。因此，更好地了解星际尘埃对天体物理学有着广泛的影响。小粒子的光学在许多科学领域都很重要，包括大气科学、海洋科学、行星科学、燃烧科学和纳米粒子研究。用户已经在所有这些领域应用了DDSCAT。将继续改进DDSCAT，并通过万维网提供。该研究项目包括研究生部分，并期望本科生参与。因此，拟议的研究有助于培养未来的科学家。＊＊＊

Our knowledge of interstellar dust is derived primarily from the absorption, scattering, and emission of electromagnetic radiation by dust grains. To test a dust model, or interpret observations, we must be able to calculate the scattering and absorption properties of model grains. In addition, scattering and absorption of light exerts forces and torques on dust grains, with important dynamical consequences. Dr. Bruce Draine will direct a project to further develop the discrete dipole approximation as a technique for calculating scattering and absorption by grains at wavelengths from the ultraviolet to the infrared. Efforts will be made to improve the fundamental approximations, and also to implement algorithmic improvements to accelerate the required numerical calculations. Dr. Draine will include these improvements in a new release of DDSCAT, a publicly-available software package for calculating electromagnetic scattering and absorption by general target shapes. A separate code will be developed to use anomalous diffraction theory to calculate scattering and absorption of X-rays by grains with arbitrary shape. This code will also be made publicly-available. These codes will be applied to various grain geometries, including fluffy aggregates and irregular compact shapes, to calculate absorption and scattering at wavelengths from infrared to X-rays. The objective is to investigate the possibility that such shapes may be representative of interstellar dust. Improved grain models will be developed, based on observational constraints including infrared emission and wavelength dependent extinction. Microwave emission from dust has been detected, and observational knowledge of the spectrum is increasing. This intensity and spectrum of this emission appears to be consistent with rotational emission from rapidly-rotating very small dust particles. Dr. Draine will study the rotational dynamics and electric dipole emission from very small grains with the aim of producing models that are in better agreement with recent observations, including observed regional variations in the microwave emission. The polarization of starlight by aligned dust grains has been known for more than half a century, but has not yet been satisfactorily accounted for. Torques exerted by starlight are known to play a major role in the grain dynamics. This work will include an extensive study of the rotational dynamics of irregular grains subject to both starlight torques and internal thermal fluctuations. The objective is to determine whether, with the grain dynamics as we now understand it, a population of irregular dust grains illuminated by anisotropic starlight will become partially-aligned, with the degree of alignment vs. grain size consistent with observations of the wavelength-dependence of polarization. If the grain model does develop alignment consistent with observations, the enigma of aligned interstellar grains will at last be solved. Dust plays an important role in the thermo-, chemo-, and hydrodynamics of the interstellar medium and the formation of planets and stars, it attenuates the optical and ultra violet spectra of galaxies, and it emits from microwave to I-band. A better understanding of interstellar dust therefore has broad impact across astrophysics. The optics of small particles is important in many scientific fields, including atmospheric science, ocean science, planetary science, combustion science, and nanoparticle studies. DDSCAT has already been applied by users in all of these areas. DDSCAT will continue to be improved and made available via the WWW. The research program includes a graduate student component, and undergraduate involvement is anticipated. The proposed research thus contributes to training future scientists. ***