5.2. Nano Silicate Grains
If nano silicate grains are present in the ISM, single-photon heating by starlight (see Section 3) will cause these grains to radiate in the 10 µm feature (see, e.g., Draine & Anderson 1985, Li & Draine 2001a). The absence of a 10 µm emission feature in IRAS spectra led Désert et al. (1986) to conclude that not more than 1% of Si could be in a < 15 Å silicate grains, and on this basis recent grain models (see Li & Draine 2001a and references therein) have excluded silicate nanoparticles as a significant grain component. Nondetection of the 10 µm silicate emission feature in the diffuse ISM by ISO (Mattila et al. 1996) and the IRTS (Onaka et al. 1996) appeared to confirm this conclusion.
However, the presence of nano silicate grains can not be ruled out since the 10 µm silicate emission feature may be hidden by the dominant PAH features. Li & Draine (2001a) have placed quantitative upper limits on the abundances of both amorphous and crystalline silicate nanoparticles by calculating spectra for such tiny grains heated by starlight, and comparing to measurements of the IR emission of the diffuse ISM by IRTS (Onaka et al. 1996) and by the DIRBE instrument on the COBE satellite. The interstellar extinction curve and the 10 µm silicate absorption profile are also invoked to provide further constraints. It was found that, contrary to previous work, as much as ~ 10% of interstellar Si could be in a 15 Å silicate grains without violating observational constraints, and not more than ~ 5% of the Si can be in crystalline silicates (of any size).