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

HofmeisterAST-9805924有关星际尘埃粒子的信息涉及银河系的起源和过程的基本问题。第一个问题是通过比较天文红外光谱和固体的实验室数据来识别“星尘”。尽管概念很简单，但实现起来却是有问题的。例如，冷碳星红外光谱中的尘埃特征被解释为六方菱面体碳化硅。这一结果与陨石中的太阳系前碳化硅颗粒完全为立方多晶型的说法不符。这种差异是根本的，因为这些恒星是产生尘埃的主要来源。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，星际碳化硅是立方的，就像在陨石中一样。由于尚未在陨石中发现太阳系前硅酸盐，正确的鉴定至关重要。将建立一个不含矩阵的红外数据库，该数据库可以直接与天文尘埃特征进行比较，这些特征不仅来自遥远的恒星，而且来自彗星等近距离天体。薄膜方法提供了随机取向的粒子，因此是“平均”消光系数，但是定量的。将收集从远红外到可见光波长的吸收数据，重点是天文学、遥感中经常使用的光谱区域，以及预计星尘中相当丰富的物质。通过干涉条纹确定薄膜厚度将得到峰高的质量依赖关系，并指示适合于大块性质的晶体尺寸。将从各种行星材料(例如硅酸盐、难熔物质以及陨石和星际尘埃颗粒中发现的相)收集100多个完整的薄膜光谱。材料将通过咨询该领域的其他科学家来选择。然后将这些数据与恒星尘埃的天文光谱进行比较。对数据库来说，同样重要的是了解产生红外光谱中吸收和散射特征的物理过程。MIE散射理论将进行修改，以考虑体吸收过程的存在，并通过与选定相(例如，碳化硅、二氧化硅、橄榄石和伽马特)的反射、薄膜和色散数据进行比较来测试该模型。分散体中的颗粒尺寸将被限制在适合表面效果的尺寸，即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不同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，如含水物种的有序性或浓度，没有考虑不可避免地存在的散射成分，也没有考虑到大于皮肤深度三倍的颗粒可以伪造峰形。这项研究工作是一个涉及国际合作的新的跨学科方向。与系内活跃研究领域的紧密联系将有助于重建研究生课程。数学和物理科学局的多学科活动办公室和国际项目部为该奖项提供资金。*

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.***