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The 3.3 Micron and Extended Red Emissions in Interstellar Clouds: Further Evidence for Carbon Nanoparticles

The excitation of photoluminescence (PL) and infrared emission from carbon nanoparticles by the absorption of photons from the interstellar radiation field is discussed using a model based on energy conservation. It is shown that the partitioning of energy between PL and internal excitation can be used to constrain the type and size of particles responsible for the extended red emission (ERE) and to relate these particles to sources of the aromatic infrared (AIR) emission features. This analysis demonstrates that the ERE (λ ≳ 600 nm) is likely emitted by particles containing ≈50 carbon atoms and that such particles are not strong emitters at 3.3 μm. It is found that only particles that do not have high PL emission efficiency can radiate significantly at 3.3 μm, and, as a result, ERE and 3.3 μm emission will not generally be cospatial. This effect is greatest in low-excitation objects. Application of this model to ERE and AIR emission in the Red Rectangle and elsewhere provides an estimate of PL efficiency that is consistent with that from polycyclic aromatic hydrocarbon and hydrogenated amorphous carbon materials. This efficiency is typically 0.1 but can be much less in regions where the hardness of the UV radiation field is reduced. An analysis of the photochemical processes involved in this conversion shows that ERE will not be produced in objects where Teff ≤ 104 K. Extended emission at longer wavelength is discussed, and it is suggested that this occurs from carbon nanoparticles with more than 50 atoms. In particular, long-wavelength bands at 1.15 and 1.5 μm detected by Gordon and coworkers are consistent with PL emission from carbon nanoparticles having ≈70 and ≈120 carbon atoms, respectively. These quantitative results provide further support for the presence of carbon nanoparticles in the interstellar medium and their assignment as the source of the extended red and AIR emissions as well as the 217.5 nm absorption band.