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T.L. Smith, A.N. Witt (U. Toledo), K.D. Gordon (LSU)
Extended red emission (ERE) is a broad, structureless emission band appearing between 540-940 nm, attributed to photoluminescence (PL) by some component of dust grains. The presence of ERE has been detected in many dusty astrophysical environments such as in the diffuse galactic ISM, high-galactic latitude cirrus clouds, reflection and planetary nebulae, H II regions, a nova and in galaxies (M82, NGC 4826). The central wavelength of the ERE band does vary from object to object and even within the same object. We calculate a lower limit to the efficiency of the ERE emission for each object, defined as \begin{eqnarray} \eta = # ~ERE~photons~emitted/# ~UV/visible~photons~absorbed \end{eqnarray} We estimate the product of the density of the radiation field and the dust column density present, the spectral energy distribution (SED) of the stellar source(s), the local opacity law and the grain albedo. A range of \eta from ~ 26.5% to ~ 0.06% was found. We interpret this range to reflect large variations in the abundance of the ERE agent, while the intrinsic efficiency of the process can in fact be very high, e.g. ~ 50% everywhere. The results indicate that a UV-soft SED, low density radiation field leads to a bluer ERE peak wavelength and a higher PL efficiency. We attribute this to the survival of very small nanometer-sized grains which are very efficient at PL due possibly to quantum confinement effects. In UV-hard radiation fields of high density, the smallest nanoparticles suffer photodissociation, resulting in a shift of the PL peak to longer wavelengths.
This work has been supported by grants from NASA, which we acknowledge with gratitude.
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