Circumstellar carbonaceous material associated with late-type dusty WC Wolf-Rayet stars

We have studied the 5-8.5 mum infrared spectra of the late-type Wolf-Rayet stars WR 118, WR 112, and WR 104, the WN star WR 147, the B5 hypergiant Cygnus OB2 No. 12, and the Galactic center luminous blue variable Pistol Star using the Short Wavelength Spectrometer on the Infrared Space Observatory. We attribute an absorption feature at 6.2 mum in the spectra of WC stars to amorphous carbon dust. This absorption feature is not detected in the diffuse interstellar medium toward the WR 147, Cyg OB2 No. 12, or the Pistol Star, and therefore we suggest that it is circumstellar in nature. In addition, we detect a broad absorption feature extending from approximately 6.5 to 8 mum. We tentatively attribute this absorption to the C-C stretching modes that accompany the 6.2 mum band in aromatic materials. Our analysis of the 6.2 mum absorption profile suggests that the dust grains have to be rather large (similar to1 mum) and point toward dense clumps as the sites of dust formation.