The Near‐Infrared Interstellar Silicate Bands and Grain Theories

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.