On ultrasmall silicate grains in the diffuse interstellar medium

The abundance of both amorphous and crystalline silicates in very small grains is limited by the fact that the 10 μm silicate emission feature is not detected in the diffuse interstellar medium (ISM). On the basis of the observed IR emission spectrum for the diffuse ISM, the observed ultraviolet extinction curve, and the 10 μm silicate absorption profile, we obtain upper limits on the abundances of ultrasmall (a ≲ 15 Å) amorphous and crystalline silicate grains. Contrary to previous work, as much as ∼10% of interstellar Si could be in a ≲ 15 Å silicate grains without violating observational constraints. Not more than ∼5% of the Si can be in crystalline silicates (of any size).