The Near‐Infrared Interstellar Silicate Bands and Grain Theories

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Abstract
With nonporous spherical grains, the interstellar 9.7 and 18 μm silicate bands are observed to be too strong to be compatible with laboratory silicates if the interstellar medium has an abundance of heavy elements that is substantially less than solar, as indicated by a number of recent observations. The dilemma can be resolved if the silicate grains are composite, fluffy, with ≥25% vacuum, and spheroidal with axis ratios greater than 2. The presence of carbon in the composite grains does not affect the silicate features much, but vacuum is important. General problems of grain models are discussed. The strong possibility that the interstellar medium has a composition with a heavy element content less than solar is briefly reviewed, and the importance of interstellar sulfur is emphasized, since it might provide a reason for believing that the interstellar medium has a solar composition. It is shown that there is a fundamental problem regarding providing the 2175 Å feature via any form of carbon: if the aromatic carbon transition in this wavelength region is strong, the central wavelength and form of the feature are sensitive to the shape distribution of the grains, or to the size distribution and states of ionization of polycyclic aromatic hydrocarbons (PAHs) or similar hydrocarbons. If the transition is weak, such as in more disordered carbon ("amorphous" carbon or material resembling coal), then the transition is independent of size and shape distributions, but too much C is required. The general viability of composite (fluffy) grains is considered in the light of recent criticism, and two unresolved problems with the silicate features are mentioned.