1995A&A...293..559P

We contest the ability of the "coal model" (Papoular et al. 1989A&A...217..204P, 1991A&A...247..215P, 1993 (in press)) of interstellar grains to explain the observed unidentified IR bands at 3.3 - 6.2 - 7.7 - 8.6 - 11.3µm. Using the optical properties of coal as measured in the laboratory by the authors of the model, we calculate the equilibrium temperature of coal grains in NGC 2023, a typical reflection nebula that exhibits these emission bands. We find T<120K and show that this temperature is significantly too low to explain the observed emission at 3.3µm. Further, we consider the emission of a typical cirrus cloud in the mid IR and conclude that the coal model cannot explain the basic observations which have led to the temperature fluctuation model, including the emission colour temperature non-depedence upon the distance from the exciting source.