INFRARED-ABSORPTION DUE TO HYDROGENATED AMORPHOUS-CARBON IN THE DIFFUSE INTERSTELLAR-MEDIUM

The infrared absorption spectrum of hydrogenated amorphous carbon (HAC) deposited at approximately 77 K is shown to provide an excellent fit to spectra of GC IRS 7 and Cyg OB2 no. 12 in the 3.4 mum band. Up to approximately 30% of available carbon may be present in this material. This form of HAC is not present in the material associated with NGC 7538 IRS 9 and other protostars, but a good fit to the short-wavelength wing of the absorption band in these objects is obtained with material that has been UV irradiated. The difference spectrum at shorter wavelengths shows the 2880 cm-1 '' diamond '' absorption reported by Allamandola et al. An absorption feature at 2880 cm-1 which may be attributable to tertiary CH in diamond-like carbon is commonly observed in HAC samples having large bandgap energy. The carbonaceous dust present in protostellar environments may therefore be a mixture of a wide bandgap diamond_like component with UV-processed diffuse cloud material.