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

Abstract

Infrared spectra of carbon-rich objects which 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 µm emission feature. To test for the presence of fullerenes, we used the position and width of the feature at 18.7-18.9 µm and examined other features at 17.4 and 6-9 µm. 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 µm 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 µm feature usually show the newly defined Class D PAH profile at 7-9 µm. These spectra exhibit unusual PAH profiles at 11-14 µm, 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 µm feature associated with 21 µm 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 µm 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 µm feature with aliphatic hydrocarbons suggest that the carrier is related to this material in some way.

Post-AGB nebular studies

“…But these lower metallicity objects harbour more prominent 11.3 microns features than in the Galaxy (where less than a handful of PNe with SiC are known). This 11.3 microns feature could be due to PAHs emission, but its shape is fully consistent with SiC (Sloan et al(2014)). SiC can indeed be less abundant but a bright SiC feature can be observed if the condensation sequence is different at low Z, leading to grain coating, with SiC forming on top of amorphous carbon grains (Sloan et al(2014); Lagadec et al 2007;Leisenring et al 2008).…”

“…Thus, Cerrigone et al(2011) proposed that it was associated with hydrocarbons. Sloan et al(2014) showed that it was always associated with two unidentified features at 15.8 and 17.1 microns. They showed that these two features were clearly linked to alkynes, confirming that the 21 micron feature is associated with hydrocarbons.…”

“…The most recent knowledge about dust evolution during the post-AGB phase has been obtained thanks to the study of objects in the Magellanic Clouds. A very thorough work by Sloan et al(2014) using infrared spectra from the Spitzer Space Telescope has identified 5 classes of objects clearly clustered in color-color diagrams. The difference of chemistry is thus likely linked to a difference in density that could be linked to the presence of discs/torii.…”

Luminosities and mass-loss rates of Local Group AGB stars and red supergiants

“…Sources with crystalline silicates in the SMC 30788 R. Sahai Sloan et al (2014) Embedded carbon stars and post-AGB objects in the LMC 40159 A. Tielens Kemper et al (2010), Woods et al (2011) Filling colour-colour and colour-mag. space in the LMC 40650 L. W. Looney Gruendl et al (2008) YSOs (and some deeply embedded carbon stars) in the LMC 50167 G. Clayton RSGs in the LMC and SMC 50240 G. C. Sloan…”

Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups: Intensity Scaling for the C–H Stretching Modes and Astrophysical Implications

“…The 3.4 µm feature is generally thought to arise from the C-H stretching vibration of aliphatic hydrocarbon materials, while the 3.3 µm feature is due to the C-H stretching mode of aromatic hydrocarbons. Also detected in some UIE sources are the aliphatic C-H deformation bands at 6.85 and 7.25 µm (see Sloan et al 2014, and see Table 3 in Yang et al 2016a for a summary).…”