THERMAL EFFECTS IN CARBONACEOUS DUST

The temperature dependence of optical and IR absorption in hydrogenate d amorphous carbon (HAC) shows that the concentration of aromatic (CH) and aliphatic (CHn, where n = 2, 3) groups together with the bandgap energy E(g) is a strong function of temperature. The rate constant, k( 1), describing the closure of the bandgap in HAC, is found to be k(1) similar to 6.8(s(-1)) exp[-(8000 +/- 2000)/T], and the elimination of H from HAC is also described by k(1). At dust temperatures, between 40 and 70 K, CH and CHn concentrations in HAC are enhanced by reaction w ith H atoms liberated from traps with binding energies 0.05-0.2 eV. I show that the ratio of optical depth for CH (3.3-mu m) and CHn (3.4-mu m) absorbers, tau (3.3)/tau(3.4), is greater than or similar to 1 for dust that has been heated to 50-60 K for an extended time, but is oth erwise small in unheated dust. These data also show that partial graph itization of HAC can occur in sources where the dust temperature T sim ilar to 200K. The release of partially graphitized 0.001-0.002 mu m ar omatic clusters in shocks may be the initial stage in the creation of small graphite grains.