▪ Abstract This review surveys the observed properties of interstellar dust grains: the wavelength-dependent extinction of starlight, including absorption features, from UV to infrared; optical lum...

https://www.ifa.hawaii.edu/users/reipurth/reviews/draine.pdf

Interstellar dust grains

The profiles, widths and positions of the Diffuse Interstellar Bands (DIB’s) are discussed. It is shown that the carriers of the DIB’s are molecular rather than impurities in grains. Polycyclic Aromatic Hydrocarbons (PAH’s) are attractive candidates for the DIB’s, in view of their likely abundance, their extreme stability, and the fact that these molecules, and their corresponding radicals, show visible transitions.

/pdf/polycyclic-aromatic-hydrocarbons-and-the-diffuse-53yjetnijz.pdf

Polycyclic aromatic hydrocarbons and the diffuse interstellar bands

The unidentified diffuse interstellar bands as evidence for large organic molecules in the interstellar medium.

Interstellar dust has a major effect on the Universe and our view of it by strongly affecting the physical and chemical processes within the interstellar medium. Its properties are best determined by the wavelength dependence of its extinction (scattering plus absorption), reflection from nearby stars, emissions (especially in certain near-infrared bands, along with continuum), and polarization of starlight by aligned grains. There are other clues as well, such as the depletion of certain elements from the gas phase onto the grains. The extinction varies strongly with direction, typically with the outer parts of molecular clouds having less extinction in the ultraviolet (relative to the visual) than the diffuse interstellar medium. There is a rather well defined one-parameter mean extinction law extending over the whole range of wavelengths for which observations are available. There are several rival theories regarding the nature of the grains and of their evolution. Most have a common base: that amorphous silicate materials and hydrogenated carbon are almost surely important constituents of grains. Various ideas regarding the arrangements are reviewed.

http://iopscience.iop.org/article/10.1088/0034-4885/56/5/001/pdf

Observations and theories of interstellar dust

Consideration is given to the work of Purcell and Pennypacker (1973) where a dielectric particle is taken to be an aggregate of N polarizable elements mounted on a cubic lattice. The simultaneous equations which result from the scattering problem are presented. This theory has been discussed in the case of nonspherical and inhomogeneous objects whose dimensions are smaller than or comparable to the wavelength of incident light. A more precise numerical treatment is derived for further progress. The variational principle is invoked and the practical limit for the current version of the scheme is a dipole array on the order of 10,000 atoms. Limits to the scattering parameter due to the phase difference between neighboring atoms are discussed.

/pdf/variational-principle-for-scattering-of-light-by-dielectric-segorh6lf7.pdf

Variational principle for scattering of light by dielectric particles.

The optical properties of grains with resonant impurities are treated, using a model which takes account of the interaction of the resonant atom with the host material that surrounds it. Such grains are often suggested as the origin of the diffuse interstellar bands. With this ''embedded-cavity model,'' extinction cross sections, linear polarization, and circular polarization are found analytically for spheroidal particles that are small compared with the wavelength. This treatment provides an effective bulk dielectric constant for the composite material that can be used to calculate the optical properties of grains of arbitrary size compared with the wavelength and arbitrary shape. The results differ somewhat from previous predictions, at least for particles small compared with the wavelength. The frequency of peak absorption is shifted, and the absorption exhibits an asymmetry in the sense that it is anomalously weaker on the red side of the line, thus appearing as a ''red emission wing.''

E. M. Purcell

Papers

A Model for the Optical Behavior of Grains with Resonant Impurities