Are Silicon Nanoparticles an Interstellar Dust Component?

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Abstract
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 ηPLf0, where ηPL < 1 is the photoluminescence efficiency for a neutral SNP and f0 ≤ 1 is the fraction of SNPs that are uncharged. For foreground extinction A0.68 μm = 1.2 mag, we find ηPLf0 > 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 ηPLf0 = 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.