The coal model applied to the Red Rectangle and the Galactic IR emission

Thanks to the knowledge of the spectral absorptivity of coal over a wide wavelength range, the coal model for interstellar dust is now in a favourable position to allow many quantitative comparisons with astronomical data. In this paper, we present two examples of such comparisons which are particularly relevant to the issue of the excitation mechanism of the UIBs (unidentified IR bands). we compare the predicted spatial distribution of the UIB emission in the Red Rectangle with the highly spatially resolved observations performed by Bregman et al. (Astrophys. J., 411, 794–796, 1993). It is shown that the two different spatial distributions observed at 3.3 and 11.3 μm are easily understandable in the frame of coal grain emission at thermal equilibrium in a decreasing radiation field. we synthesize the average emission spectrum of the Galaxy from 2 to 400 μm, taking into account all the available observations. It is found that the near-IR Galactic emission can be attributed to a very small amount of warm coal-like grains (300 K) in the immediate vicinity of excited nebulae, while far-IR radiation is emitted by cold coal- and silicate-like grains in the diffuse ISM (interstellar medium). h cases considered here, steady-state heating of coal-like grains is compatible with all the observations.