HYDROGENATED AMORPHOUS-CARBON GRAINS AND THE 2175-A INTERSTELLAR HUMP

In this paper we present the first extinction spectra of carbon grains condensed in a hydrogen atmosphere to simulate the formation of dust particles in hydrogen-rich astrophysical environments. In order to assess the influence of hydrogen during grain formation unambiguously, experiments were performed in hydrogen-poor (Ar) and hydrogen-rich (H-2) atmospheres allowing the carbon vapor to condense homogeneously into grains. The experimental data confirm the theoretical predictions on the lack of any feature in the ultraviolet spectra of fully hydrogenated amorphous carbon grains and clearly show the relevance of the different atmospheres during the condensation process. Our results support the idea that grains observed in the interstellar medium, which show a strong 2175 angstrom feature, have undergone some processing subsequent to their formation.