3.2. Dust Emission

3.2.1. Dust Luminescence: The "Extended Red Emission"

"The ERE has become an important observational aspect
of interstellar grains that future models need to reproduce."

-- A.N. Witt [2000]

First detected in the Red Rectangle (Schmidt, Cohen, & Margon 1980), "extended red emission" (ERE) from interstellar dust consists of a broad, featureless emission band between ~ 5400 Å and 9000 Å, peaking at 6100 _p 8200 Å, and with a width 600 Å FWHM 1000 Å. The ERE has been seen in a wide variety of dusty environments: the diffuse ISM of our Galaxy, reflection nebulae, planetary nebulae, HII regions, and other galaxies (see Witt, Gordon, & Furton 1998 for a summary). The ERE is generally attributed to photoluminescence (PL) by some component of interstellar dust, powered by UV/visible photons. The photon conversion efficiency of the diffuse ISM has been determined to be near (10 ± 3)% (Gordon et al. 1998; Szomoru & Guhathakurta 1998) assuming that all UV/visible photons absorbed by interstellar grains are absorbed by the ERE carrier. The actual photoluminescence efficiency of the ERE carrier must exceed ~ 10%, since the ERE carrier cannot be the only UV/visible photon absorber.

Various forms of carbonaceous materials - HAC (Duley 1985; Witt & Schild 1988), PAHs (d'Hendecourt et al. 1986), QCC (Sakata et al. 1992), C₆₀ (Webster 1993), coal (Papoular et al. 1996), PAH clusters (Allamandola, private communication), carbon nanoparticles (Seahra & Duley 1999), and crystalline silicon nanoparticles (Witt et al. 1998; Ledoux et al. 1998) - have been proposed as carriers of ERE. However, most candidates appear to be unable to simultaneously match the observed ERE spectra and the required PL efficiency (see Witt et al. 1998 for details).

Although high photoluminescence efficiencies can be obtained by PAHs, the lack of spatial correlation between the ERE and the PAH IR emission bands in the compact HII region Sh 152 (Darbon et al. 2000), the Orion Nebula (Perrin & Sivan 1992), and the Red Rectangle (Kerr et al. 1999), and the detection of ERE in the Bubble Nebula where no PAH emission has been detected (Sivan & Perrin 1993) seem against PAHs as ERE carriers.

Seahra & Duley (1999) argued that small carbon clusters were able to meet both the ERE profile and the PL efficiency requirements. However, this hypothesis appears to be ruled out by non-detection in NGC 7023 of the 1 µm ERE peak (Gordon et al. 2000) predicted by the carbon nanoparticle model.

Witt et al. (1998) and Ledoux et al. (1998) suggested crystalline silicon nanoparticles (SNPs) with 15Å - 50Å diameters as the carrier on the basis of experimental data showing that SNPs could provide a close match to the observed ERE spectra and satisfy the quantum efficiency requirement. Smith & Witt (2002) have further developed the SNP model for the ERE, concluding that the observed ERE in the diffuse ISM can be explained with Si/H = 6 ppm in SiO₂-coated SNPs with Si core radii a 17.5 Å.

Li & Draine (2002b) calculated the thermal emission expected from such particles, both in a reflection nebula such as NGC 2023 and in the diffuse ISM. They found 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 < 0.2ppm Si in Si / SiO₂ SNPs. The ERE emissivity of the diffuse ISM appears to require > 15 ppm ( 42% of solar Si abundance) in Si/SiO₂ SNPs. In comparison with the predicted IR emission spectra, they found that the DIRBE (Diffuse Infrared Background Experiment) photometry appears to rule out such high abundances of free-flying SNPs in the diffuse ISM. Therefore they concluded that if the ERE is due to SNPs, they must be either in clusters or attached to larger grains. Future observations by SIRTF will be even more sensitive to the presence of free-flying SNPs.