Interstellar grains The interaction of light with a small-particle system

Abstract

Abstract Interstellar dust is suggested to be one of the most thoroughly studied of all small-particle systems, whose characteristics have in some ways not been duplicated in our laboratories. The particles are maintained in complete isolation from one another, in ultra-high vacuum and at low temperature, and have been studied spectroscopically from far infra-red to far ultra-violet. Optical properties of the interstellar dust which are surveyed include effects interpreted as due to surface plasmon bands in the ultra-violet, surface phonon bands and other vibrational absorption bands in the infra-red, linear and circular polarization caused by aligned particles, impurity absorption, and defect absorption possibly caused by radiation damage. A discussion of the possible origins for interstellar grains leads to speculation on their relationship to certain meteorites. Absorption and scattering effects from small particles in general are surveyed by way of Mie calculations for a representative insulator, MgO, and a representative metal, Mg. Spectral features revealed by the calculations from far infra-red to far ultra-violet are discussed briefly. Methods of measuring optical constants, necessary for small-particle calculations, are surveyed, and results for several solids of possible importance in astronomy are discussed. Collective resonances associated with plasmons and phonons in small particles are treated, and the effect of particle shape is illustrated with several experimental examples. Various non-collective resonance effects in small particles are presented and situations in which bulk optical constants may not be appropriate are reviewed. Applications are made of all these effects to observed features from interstellar grains. Special emphasis is given to a discussion of the 39 unidentified interstellar bands in the visible spectral region, which comprise the longest-standing spectroscopic mystery in astronomy.