POWRE: Construction of an Infrared Spectroscopic Database for Direct Comparison with Astronomical Observations of Stardust

关于星周尘埃粒子的信息涉及银河系起源和过程的基本问题。一级问题是通过比较天文红外光谱与固体的实验室数据来识别“星尘”。虽然概念很简单，但实现起来有问题。例如，冷碳星红外光谱中的尘埃特征被解释为六边形-菱形SiC。这一结果与前太阳碳化硅颗粒在陨石中完全是立方多晶的鉴定不一致。这种差异是根本性的，因为这类恒星是主要的尘埃制造者。0.1微米颗粒的薄膜或原始色散光谱数据与碳星光谱数据的比较表明，星周碳化硅是立方的，就像在陨石中一样。正确的鉴定是至关重要的，因为太阳前硅酸盐尚未在陨石中发现。将建立一个无矩阵的红外数据库，可以直接与天文尘埃特征进行比较，不仅来自遥远的恒星，也来自附近的物体，如彗星。薄膜法提供随机取向的粒子，因此“平均”消光系数，但是定量的。吸收数据将从远红外线到可见光波段收集，重点是天文学、遥感中经常使用的光谱区域，以及星尘中预计相当丰富的物质。通过干涉条纹测定薄膜厚度将产生峰高的质量依赖性，并指示适合体性质的晶体尺寸。超过100个完整的薄膜光谱将从各种行星材料（例如，硅酸盐，耐火物质，在陨石和行星际尘埃颗粒中发现的相）中收集。材料将通过咨询该领域的其他科学家来选择。然后将这些数据与星尘的天文光谱进行比较。对数据库同样重要的是理解在红外光谱中产生吸收与散射特征的物理过程。米氏散射理论将被修改，以解释体吸收过程的存在，并通过与反射、薄膜和来自选定相（例如，SiC、SiO2、橄榄石和gamet）的色散数据进行比较来测试模型。分散体中颗粒的大小将被限制为适合表面效应的尺寸，0.05微米。修订后的理论应允许从实验室吸收数据中计算具有可变数量的散射和吸收成分的光谱。该项目的成果将在天文学、行星科学和遥感领域发挥作用。这对矿物学也很重要，因为目前从色散光谱中提取的物理性质，如含水物种的排序或浓度，没有考虑到不可避免地存在的散射成分，也没有考虑到尺寸大于皮肤深度三倍以上的颗粒可能伪造峰剖面。本研究是一个涉及国际合作的跨学科新方向。与该系活跃研究领域的紧密联系将有助于重建研究生课程。数学和物理科学理事会多学科活动办公室和国际项目司为该奖项提供资金

HofmeisterAST-9805924Information on circumstellar dust particles pertains to basic questions of origin and process in the galaxy. A first order problem is identification of "stardust" through comparison of astronomical infrared (IR) spectra to laboratory data on solids. Although the concept is straightforward, implementation is problematic. For example, dust features in the IR spectra of cold, carbon stars have been interpreted as hexagonal-rhombohedral SiC. This result disagrees with identification of presolar SiC grains in meteorites as exclusively the cubic polymorph. The discrepancy is fundamental because such stars are major dust producers. The comparison of data from thin film, or raw dispersion spectra, for O.1 micron grains to the data of carbon star spectra shows that circumstellar SiC is cubic, just as it is in meteorites. Correct identification is crucial as presolar silicates have not yet been found in meteorites. A matrix-free IR database will be constructed that can be directly compared with astronomical dust features, not only from distant stars, but also from near-by objects such as comets. The thin film method provides randomly orientated particles, and hence "average" extinction coefficients, yet is quantitative. Absorption data will be collected from the far-IR to the visible wavelengths, with emphasis on spectral regions frequently used in astronomy, remote sensing, and on materials expected to be fairly abundant in stardust. Determination of film thickness through interference fringes will yield the mass dependence of peak heights and indicate crystal sizes appropriate for bulk properties. Over 100 complete thin film spectra will be collected from various planetary materials (e.g., silicates, refractory substances, and phases found in meteorites and interplanetary dust particles). Materials will be selected by consulting other scientists in the field. The data will then be compared to astronomical spectra of stardust.Of equal importance to the database is the understanding the physical processes producing absorption vs. scattering features in IR spectra. Mie scattering theory will be modified to account for the presence of bulk absorption processes, and to test the model through comparison with reflection, thin film, and dispersion data from selected phases (e.g., SiC, SiO2, olivine and gamet). The size of grains in the dispersions will be limited to sizes appropriate for surface effects, 0.05 micron. The revised theory should allow calculation of spectra with variable amounts of scattering and absorption components from laboratory absorption data. The results of the project will be useful in the fields of astronomy, planetary science, and remote sensing. It will also be important to mineralogy, in that physical properties such as ordering or concentrations of hydrous species are currently extracted from dispersion spectra without accounting for the component of scattering inevitably present and without considering that particles with sizes more than three times larger than the skin depth can falsify peak profiles.This research effort is a new, interdisciplinary direction involving an international collaboration. Strong links with active research areas in the department will help to rebuild a graduate program.The Office of Multidisciplinary Activities in the Directorate of Mathematical and Physical Sciences and the Division of International Programs are providing the funds for this award.***