EFFECT OF VARIABLE GRAPHITIC AND DIAMOND-LIKE CONTENT ON THE TEMPERATURE OF CARBONACEOUS DUST

The equilibrium temperature of composite carbon grains is shown to be a sensitive function of the ratio of aromatic (sp2) to diamond (sp3) b onded carbon in the grain. Calculations have been carried out for grai ns with radii 0.001 less-than-or-equal-to a less-than-or-equal-to 0.1 mum and compositions that consist of mixtures of aromatic carbon, diam ond-like carbon, polymeric carbon and voids. An effective medium appro ximation has been used to estimate dielectric constants for such mater ials. Our results indicate that the addition of a small amount of arom atic carbon results in a dramatic reduction in grain temperature. Grai n temperatures as low as 11 K are found under certain conditions when A(V) = 0. The relevance of this to the condensation of solid CO on dus t grains in clouds is briefly discussed.