Carbon grains produced in partially hydrogenated atmospheres and their astrophysical relevance

The presence of carbonaceous particles, both in circumstellar regions and the interstellar medium, now seems quite well accepted. The exact nature of the carbon grains, however, is still controversial and the subject of a wide and live debate from a theoretical as well as an experimental point of view. Recently in a series of works we have shown that the carbon grains produced and studied in our laboratory may be good candidates as laboratory dust analogs. In particular, by producing and thermally processing fully hydrogenated amorphous carbon grains, we have shown that the heat treatment of the particles can be a reasonable simulation of the grain processing active in space as modeled by other authors. The heat treatment decreases the hydrogen content of the grains which, for this reason, change their internal structure and their optical properties in the 190-260 nm range. An ultraviolet extinction bump, absent in the fully hydrogenated carbon grains, appears and becomes more and more pronounced as the annealing temperature increases, while at the same time the position of the peak shifts toward longer wavelengths. In this work, we show that similar results can be obtained if carbon grains are produced by direct condensation of the carbon vapors in partially hydrogenated atmospheres. By changing the contribution of the hydrogen gas to the total pressure inside the condensation chamber, the resulting grains exhibit an extinction peak whose wavelength position shifts toward longer wavelengths as the hydrogen abundance decreases. The results presented in this paper may explain the spectral differences in the ultraviolet extinction observed toward some circumstellar regions without invoking any kind of dust evolution and, at the same time, confirm the importance of the hydrogen abundance during the dust formation processes. This means that our findings may have important consequences for the nucleation/condensation theories and evolution of dust grains as well as for the study of dust formation regions.