Circumstellar Carbonaceous Material Associated with Late-Type Dusty WC Wolf-Rayet Stars

Abstract

We have studied the 5-8.5 μm 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 μm 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 μm. We tentatively attribute this absorption to the CC stretching modes that accompany the 6.2 μm band in aromatic materials. Our analysis of the 6.2 μm absorption profile suggests that the dust grains have to be rather large (~1 μm) and point toward dense clumps as the sites of dust formation. Based on observations made with the Infrared Space Observatory, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.