EXCITATION OF EXTENDED RED EMISSION AND NEAR-INFRARED CONTINUUM RADIATION IN THE INTERSTELLAR MEDIUM

Many small molecules including carbon clusters emit blackbody radiation in the visible spectrum when their internal temperature, T, is raised above 2000 K by photoabsorption. Blackbody emission is known to be the dominant cooling mechanism for small dehydrogenated carbon molecules for 1500< T< 3000 K. The condition that T > 2000 K would be met by interstellar molecules containing <= 28 carbon atoms, heated by energetic photons from the interstellar radiation field. It is shown here that thermal emission will augment photoluminescent emission in extended red emission (ERE) sources when the UV radiation field is enhanced. In particular, this mechanism provides a simple explanation for observations that show that only stars with T(eff) > 7000 K excite the ERE. The observation by Witt et al. that photons with energies> 10.5 eV are required for the onset of ERE emission can then be interpreted as the condition for the initiation of thermal emission at visible wavelengths. These observational requirements have been combined with laboratory and theoretical data to constrain the emitters of the ERE to dehydrogenated carbon molecules, C(N) with 20 <= N <= 28 atoms. The composition and structure of these molecules is discussed and IR band energies for several possible C(N) species are provided. These molecules are stable against photodissociation in the interstellar radiation field. It is also shown that dimers of these molecules, (C(N))(2), may be the species that give rise to the near-infrared continuum first detected by Sellgren. A new effect that might be significant under interstellar conditions involving unimolecular rearrangement reactions in thermally excited molecules is also discussed.