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. ***
