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Because the division consists of many very active commissions, most activities are included in the reports of the individual commissions. This report highlights a small subset of the major achievements that are covered in detail in the reports by the commissions. Some administrative activities of the division and reports of the divisional working groups and committees are also included as subsequent sections of this divisional report.
We use the DDA method to study scattering properties of aggregates. Based on this aggregate model, the temperatures of cometary dust particles will be close to those of the equivalent total volume sphere when the aggregate porosity is ≤ 0.64.
Comet dust consists primarily of silicate and carbonaceous material. The carbonaceous material is not yet well-characterized. The 3.36 μm emission feature arises primarily from gas phase molecules, while a small feature at 3.29 μm may be due to aromatic hydrocarbons in the grains. Olivine, a high temperature condensate, is present in some comets. Polarization and albedo maps have shown that the coma is not uniform; an inner halo of large particles may be present.
Study of the dust in circumstellar disks around young stars is currently an extremely active area in astronomy. There is little doubt that accretion disks are a natural part of protostellar evolution. Much recent observational and theoretical work is giving us a clearer picture of the physical conditions in dust disks and their evolutionary progression. IRAS observations revealed that many main-sequence stars, such as p Pictoris, have circumstellar disks. But whether these disks are related to planetary formation is not yet understood.
A portion of the dust in disks around young stars ultimately may be incorporated into planetary systems. Thus, study of the dust in our own solar system complements the remote sensing of protostellar regions and aids in reconstructing the evolutionary history of the dust. Since comets formed in the cold outer regions of the solar nebula, they may contain intact interstellar grains. As the comets lose material during passage through the warm inner solar system, some of these grains will be released into interplanetary space. Technical advances now allow analysis of individual micrometer or smaller grains in interplanetary dust particles and primitive meteorite samples. Isotopic anomalies and patterns of crystal growth in these particles are yielding tantalizing clues about the interstellar material incorporated into these solar system samples.
Thermal emission from interplanetary dust is the main source of diffuse radiation at λ 5-50 μm. Analysis of infrared sky maps from IRAS and ZIP lead to the result that the average optical properties of the dust change with heliocentric distance. The present uncertainties in calibration should be resolved by COBE. Existence of a dust sublimation zone at 4 solar radii awaits confirmation at the next solar eclipse.
The infrared spectral region (1–1000 μm) is important for studies of both molecules and solid grains in comets. Infrared astronomy is in the midst of a technological revolution, with the development of sensitive 2–dimensional arrays leading to infrared cameras and spectrometers with vastly improved sensitivity and resolution. The Halley campaign gave us tantalizing first glimpses of the comet science possible with this new technology, evidenced, for example, by the many new spectral features detected in the infrared. The techniques of photometry, imaging, and spectroscopy are reviewed in this chapter and their status at the time of the Halley observations is described.
Observations of the scattered sunlight and thermal emission from cometary dust provide information on the composition and dominant size of the dust. The observations will be summarized here and compared to theoretical models for dielectric and absorbing materials, with emphasis on the thermal emission. The compatibility of the optical data with the size distribution derived from dynamical studies is discussed.
The interplanetary dust may be composed of cometary material, interstellar grains, debris from asteroidal collisions, primordial material formed by direct condensation, or contributions from all of these sources. Before we can determine the origin of the dust, we need to know its physical nature, spatial distribution, and the dynamical forces that act on the particles. The spatial distribution and dynamics are separately treated in this symposium by Roosen. We discuss here the physical characteristics of the dust particles: their size distribution, chemical composition, physical structure, and optical properties.
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