Grain temperature and infrared emission from carbon dust of mixed composition

The equilibrium temperature of carbonaceous dust grains whose composition is consistent with IR spectra of diffuse cloud and dense cloud dust has been calculated using random covalent network (RCN) solutions for amorphous dust having a mixed graphite, diamond, and polymeric hydrocarbon composition. An effective medium approximation has been adopted to describe optical and thermal constants for dust compositions consistent with IR absorption spectra. A small amount of sp(2) hybridized carbon in the form of aromatic rings is found to have a significant effect in reducing equilibrium temperature in dust with high diamond/polymer content. This formalism has also been used to calculate nonequilibrium emission spectra of very small grains (VSGs) subjected to stochastic heating in the interstellar radiation field. Such grains are found to emit strongly in sharp IR bands associated with C-H bonds at 3.4 mu m and longer wavelengths. The effect of varying graphite/diamond/hydrocarbon composition on nonequilibrium emission by VSGs can also be described using this formalism. The ratio of emission at 12 and 25 mu m is found to be high for VSGs with a large fraction of diamond or polymeric hydrocarbon but decreases dramatically for dust with a large sp(2) aromatic component.