Optical Properties of Interstellar Dust

Princeton University, Princeton NJ

Investigators

Abstract

Project Summary: Optical Properties of Interstellar Dust Dust plays an important role in the thermo-, chemo-, and hydrodynamics of the interstellar medium and in the formation of planets and stars. Dust attenuates the optical and ultraviolet (UV) spectra of galaxies, and it emits from the infrared to microwave wavelengths. A better understanding of interstellar dust has broad impact across astrophysics. Our knowledge of interstellar dust is derived primarily from remote observation of absorption, scattering, and emission of electromagnetic radiation by dust grains. In order to interpret observations and test dust models, the scattering and absorption properties of model grains need to be calculated. There are also important dynamical consequences from scattering and absorption of light because this exerts forces and torques on dust grains. In this work a number of related investigations concerning the optical properties of interstellar dust will be carried out. The investigators, Professor Draine and a graduate student, plan to develop improved methods for calculating scattering and absorption by small particles or nanostructures. They plan further develop and apply the discrete dipole approximation (DDA) as a technique for calculating scattering and absorption by irregular grains and composite grains at wavelengths from the ultraviolet to the infrared. A new approach will be pursued to determine ?surface corrected? dipole polarizabilities to improve the accuracy of the DDA. Other algorithmic improvements will also be implemented and included in a new release of the discrete dipole approximation code DDSCAT, which is a publicly-available DDA code for calculating light scattering and absorption by general target shapes. Anomalous diffraction theory will be used at X-ray energies. The initial objective will be to create a ?library? of scattering and absorption properties over a broad range of wavelengths for selected grain sizes, shapes, and compositions. Grain geometries will include grains that are built up from smaller monomers, including the ?fluffy? clusters created by the standard ?ballistic aggregation? procedure, as well as by other aggregation rules that result in clusters that are more compact, less fragile, and possibly more realistic. It is also planned to study the scattering and absorption by grains that are irregular but compact, using a variant of the ?Gaussian spheres? technique to generate random shapes. This library of scattering results will be useful in subsequent studies of light scattering and absorption and should have wide applicability. It is planned to make the library publicly available on the principal investigator?s web site. The development of an improved dielectric function for the silicate material in the diffuse ISM is planned as part of this work. The dielectric function is required to reproduce the observed interstellar silicate features in both extinction and polarization, and to be consistent with other astrophysical constraints including far-infrared and submm emission and interstellar abundances. The relationship between the silicate extinction and polarization is sensitive to grain shape and to the dielectric function. The goal is to successfully reproduce both, which will give some confidence in the resulting dielectric function and grain shape. After creating the library of scattering properties for grains of various types, models for interstellar dust will be constructed with size distributions adjusted to reproduce observations of wavelength-dependent extinction, wavelength-dependent polarization of starlight, infrared emission, and X-ray scattering properties. It is planned to do this with different grain types, including compact grains and fluffy grains. This will help to narrow down the kinds of grains (e.g., fluffy grains) that give model results compatible with observations. A study of the different cases thus will show which grain type would be ruled out as a major interstellar grain component. The proposed work will have broad interdisciplinary impact. The optics of small particles is important in many scientific fields, including atmospheric science, oceanology, planetary science, combustion science, marine biology, and nanoparticle studies. DDSCAT has already been applied by users in all of these areas. Further improvements in our ability to calculate absorption and scattering by particles and structures will be of value beyond astrophysics. The PI will continue to support the DDSCAT package and to make it publicly-available via the WWW.