Optics and Dynamics of Interstellar Dust

我们对星际尘埃的认识主要来自尘埃颗粒对电磁辐射的吸收、散射和发射。为了测试尘埃模型，或解释观测结果，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。布鲁斯德莱恩博士将指导一个项目，以进一步发展离散偶极子近似作为一种技术，用于计算散射和吸收的谷物在波长从紫外线到红外线。将努力改进基本近似，并改进算法，以加速所需的数值计算。Dr. Draine将在新版本的DDSCAT中包括这些改进，DDSCAT是一个公开的软件包，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，使用反常衍射理论计算X射线的散射和吸收的任意形状的颗粒。该代码也将公开提供。这些代码将应用于各种颗粒几何形状，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。其目的是研究这种形状可能代表星际尘埃的可能性。改进的颗粒模型将开发，根据观测的限制，包括红外发射和波长相关的消光。已经探测到尘埃的微波辐射，对光谱的观测知识正在增加。这种发射的强度和光谱似乎与快速旋转的非常小的尘埃粒子的旋转发射一致。 Dr. Draine将研究非常小的颗粒的旋转动力学和电偶极发射，目的是产生与最近的观测结果更一致的模型，包括观察到的微波发射的区域变化。半个多世纪以来，人们已经知道了排列的尘埃颗粒对星光的偏振作用，但至今还没有令人满意的解释。由星光所施加的扭矩在晶粒动力学中起着重要的作用。 这项工作将包括广泛的研究不规则颗粒的旋转动力学受到星光扭矩和内部热波动。我们的目标是确定，与颗粒动力学，因为我们现在知道它，人口的不规则尘埃颗粒照射各向异性星光将成为部分对齐，与对齐的程度与晶粒尺寸一致的观察偏振的波长依赖性。如果粒子模型确实发展出与观测一致的排列，那么星际粒子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作用，它衰减星系的光学和紫外光谱，并从微波到I波段发射。 因此，更好地了解星际尘埃对整个天体物理学具有广泛的影响。小粒子的光学特性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。DDSCAT已被用户应用于所有这些领域。DDSCAT将继续得到改进，并通过万维网提供。该研究计划包括一个研究生组成部分，并预计本科生的参与。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. ***