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 micrometer 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 ultraviolet (UV) irradiated. The difference spectrum at shorter wavelengths shows the 2880 per cm 'diamond' absorption reported by Allamandola et al. An absorption feature at 2880 per cm which may be attributable to tetiary 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.