Interstellar Dust: Optics and Models

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The intellectual merit of this research project is broad. Dust reduces the light from galaxies, sends out its own light, and plays an important role in the thermo-, chemo-, and fluid-dynamics of the matter between the star systems in a galaxy, and in the formation of planets and stars. An improved understanding of the physical properties of interstellar dust grains will have implications for studies of diverse astrophysical phenomena, from formation of stars and planets to the material near active galactic nuclei.Our knowledge of interstellar dust derives primarily from remote observations of absorption, scattering, and emission of electromagnetic radiation by dust grains. The scattering and absorption properties of dust grains are needed to interpret observations, and to test dust models. In addition, scattering and absorption of light exerts forces and torques on dust grains, with important dynamical consequences. Dr. Bruce Draine and his research team will carry out a number of investigations concerning the optical properties of interstellar dust, and development of improved tools for calculating the optical properties of small particles with irregular geometries. These tools will be used to find new state-of-the-art models (composition, shape, and size distribution) for interstellar dust that are consistent with observational constraints.The initial objective is to create a "library" of calculated cross sections for scattering and absorption over a broad range of wavelengths, for selected grain sizes, shapes, and compositions. Grain geometries will include spheroids, fluffy grains built up from random coagulation of spheres, and grains that are irregular but compact. State-of-the-art codes will be employed. The discrete dipole approximation (DDA) will be used for irregular grain geometries. Accurate calculations of absorption and differential scattering cross sections at X-ray energies will be carried out using a new code applying anomalous diffraction theory to general geometries. This code will be documented and made publicly-available (open source) on the WWW. Dr. Draine will continue to develop and support the open-source code DDSCAT for calculations using the DDA. The cross-section library will be made available on the WWW. For each grain, the team will calculate the temperature distribution function for various heating radiation fields; the library of temperature distribution functions will also be made available on the WWW. These will be valuable community resources.The calculated cross-sections and temperature distribution functions will be used to construct the best-to-date interstellar dust models with size distributions adjusted to reproduce observations of wavelength-dependent extinction, wavelength-dependent polarization of starlight, polarized infrared emission, and X-ray scattering. Models will be built with different grain shapes, including compact grains and fluffy grains. Interstellar grains may include magnetic material. The team will continue study of the intensity and polarization of the magnetic dipole radiation emitted by such grains. This magnetic dipole radiation may be important at mm and cm wavelengths, contributing a new polarized component to the "galactic foreground", affecting studies of the polarization of the cosmic microwave background.This work has broad interdisciplinary impact. Improved methods for calculating scattering and absorption by nanostructures have wide applicability, beyond astrophysics. The optics of small particles is important in many scientific fields, including atmospheric science, oceanography, planetary science, combustion science, marine biology, and nanoparticle studies. DDSCAT has already been applied by users in all of these areas. The DDSCAT package will continue to be supported by Dr. Draine and made publicly available via the WWW. The research program includes a graduate student component, and undergraduate participation is planned. The proposed research will thereby contribute to training future scientists.