THE COAL MODEL FOR THE UNIDENTIFIED INFRARED BANDS .2. THE THERMAL EMISSION MECHANISM

New spectroscopic data are presented in support of the coal model for the composition and structure of the Unidentified Infrared Band (UIB) emitters; these data were obtained by diffuse-reflectance, Fourier-transform spectroscopy of two high ranking coals of different origins in the range 2.5-20-mu-m. Pursuing the elaboration of the model, we then address the problem of the emission mechanism and consider, in particular, coal emission in thermal equilibrium. This requires a knowledge of the spectral absorptivity of coal at various temperatures and in vacuum. For this purpose, we used an original technique based on the measurement of the amplitude and phase of the sample emission (between 1.5 and 4.5-mu-m) under irradiation by a chopped beam of UV/visible light. Absorptivity curves were obtained in the range approximately 300 to approximately 600 K, from which spectral emissions were deduced; these compare reasonably well with the spectra of various astronomical sources, showing that this is the right range of grain temperatures. It is then possible to estimate the input energy flux density required to maintain a grain of size d (angstrom) at a temperature T: e.g. 2.5 10(10) d (eV cm-2 s) for T congruent-to 400 K. There is no tight constraint on the nature of the particles which have to provide that energy, but electrons are individually more efficient than photons for this purpose; for photons, the wavelength is not critical. All these properties properly fit the available data on UIB's. In particular, maps of ionized UIB-emitting regions show that the emission peaks at the interface between ionized and dissociated-hydrogen zones. The estimated power carried by electrons in this interface is adequate for heating 100 angstrom-sized grains to approximately 400 K. The recombination of H atoms on the grains can also provide some heat. The size distribution of coal-like particles in space is expected to extend down to the smallest structural entity in coal, 10-20 angstrom in size. In this limit, transient heating by single UV photons may compete with the thermal equilibrium mechanism in regions of weak ISRF.