Carbon-rich Dust Past the Asymptotic Giant Branch: Aliphatics, Aromatics, and Fullerenes in the Magellanic Clouds

Infrared spectra of carbon-rich objects that have evolved off the asymptotic giant branch reveal a range of dust properties, including fullerenes, polycyclic aromatic hydrocarbons (PAHs), aliphatic hydrocarbons, and several unidentified features, including the 21

\mu

m emission feature. To test for the presence of fullerenes, we used the position and width of the feature at 18.7-18.9

\mu

m and examined other features at 17.4 and 6-9

\mu

m. This method adds three new fullerene sources to the known sample, but it also calls into question three previous identifications. We confirm that the strong 11

\mu

m features seen in some sources arise primarily from SiC, which may exist as a coating around carbonaceous cores and result from photo-processing. Spectra showing the 21

\mu

m feature usually show the newly defined Class D PAH profile at 7-9

\mu

m. These spectra exhibit unusual PAH profiles at 11-14

\mu

m, with weak contributions at 12.7 μm, which we define as Class D1, or show features shifted to ~11.4, 12.4, and 13.2 μm, which we define as Class D2. Alkyne hydrocarbons match the 15.8

\mu

m feature associated with 21

\mu

m emission. Sources showing fullerene emission but no PAHs have blue colors in the optical, suggesting a clear line of sight to the central source. Spectra with 21

\mu

m features and Class D2 PAH emission also show photometric evidence for a relatively clear line of sight to the central source. The multiple associations of the 21

\mu

m feature with aliphatic hydrocarbons suggest that the carrier is related to this material in some way.

Note:

30 pages, 30 figures, ApJ, in press

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