Are silicon nanoparticles an interstellar dust component?

Silicon nanoparticles (SNPs) with oxide coatings have been proposed as the source of the observed "extended red emission" (ERE) from interstellar dust. We calculate the thermal emission expected from such particles, both in a reflection nebula such as NGC 2023 and in the diffuse interstellar medium (ISM). It is shown that Si/SiO₂ SNPs (both neutral and charged) would produce a strong emission feature at 20 μm. The observational upper limit on the 20 μm feature in NGC 2023 imposes an upper limit of less than 0.2 parts per million in Si/SiO₂ SNPs. The observed ERE intensity from NGC 2023 then gives a lower bound on the product η_PL f₀, where η_PL < 1 is the photoluminescence efficiency for a neutral SNP and f₀ ≤ 1 is the fraction of SNPs that are uncharged. For foreground extinction A _0.68μm = 1.2 mag, we find η_PL f₀ > 0.24 for Si/SiO₂ SNPs in NGC 2023. Measurement of the R-band extinction toward the ERE-emitting region could strengthen this lower limit. The ERE emissivity of the diffuse interstellar medium appears to require ≳42% of solar Si abundance in Si/SiO₂ SNPs even with η _PL f₀ = 1. We predict IR emission spectra and show that DIRBE photometry appears to rule out such high abundances of free-flying SNPs in the diffuse ISM. We conclude that if the ERE is due to SNPs, they must be either in clusters or attached to larger grains.