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 ÏÏ (ERE) from interstellar dust. We calculate the thermal emission expected from such particles, both in a reÑection nebula such as NGC 2023 and in the di†use interstellar medium (ISM). It is shown that SNPs (both neutral and charged) would produce a strong emission 2 feature at 20 km. The observational upper limit on the 20 km feature in NGC 2023 imposes an upper limit of less than 0.2 parts per million in SNPs. The observed ERE intensity from NGC 2023 2 then gives a lower bound on the product where is the photoluminescence efficiency for a gPL f0, gPL\ 1 neutral SNP and is the fraction of SNPs that are uncharged. For foreground extinction f01 1 A0.68 km \ 1.2 mag, we Ðnd for SNPs in NGC 2023. Measurement of the R-band extinction gPL f0[ 0.24 2 toward the ERE-emitting region could strengthen this lower limit. The ERE emissivity of the di†use interstellar medium appears to require of solar Si abundance in SNPs even with Z42% 2 We predict IR emission spectra and show that DIRBE photometry appears to rule out such gPL f0\ 1. high abundances of free-Ñying SNPs in the di†use ISM. We conclude that if the ERE is due to SNPs, they must be either in clusters or attached to larger grains. Subject headings : dust, extinction È infrared : ISM È ISM: individual (NGC 2023) È ISM: lines and bands È reÑection nebulae