A DEHYDROGENATION STUDY OF COSMIC CARBON ANALOG GRAINS

We present the results of a systematic study of the dehydrogenation produced by thermal annealing of small (average radius [a] = 5 nn) hydrogenated amorphous carbon grains. The heat treatment of the particles can be a reasonable simulation of the grain processing active in space as modeled by Hecht (1986) and Sorrell (1990). Extinction measurements both on grains as produced and on samples annealed at fixed temperatures between 250 degrees C and 800 degrees C have been performed in the spectral range 190-800 nm. For some samples the measurements have been extended in the extreme-ultraviolet (EUV) down to 40 nm. The extinction spectra of all the samples analyzed in the EUV are characterized by a band falling around 93 nm. On the other hand, all the samples, but the hydrogenated one, show a peak in the UV region. This feature becomes more and more pronounced and shifts toward longer wavelengths as the annealing temperature increases. We assign the high- and low-energy bands, respectively, to sigma-sigma* and pi-pi* electronic transitions of the sp(2) hybridized carbon clusters forming the grains. We identify these clusters as the major factor responsible for determining the UV extinction properties of our carbon grains. The shift of the peak is interpreted in terms of a dimensional growth of the sp(2) clusters, indicating a progressive graphitization of the samples. The pi-pi* transition is observed at 259 nm in the samples annealed at 800 degrees C, in close agreement with that observed for graphite (260 nm). The results of the annealing experiment offer new hints to interpret the circumstellar and interstellar UV extinction bands. In particular, it appears possible to interpret the origin of these features in terms of pi-pi* electronic transitions in carbonaceous grains.