Infrared Study of Fullerene Planetary Nebulae

Abstract

We present a study of sixteen Planetary Nebulae (PNe) where fullerenes have been detected in their Spitzer Space Telescope spectra. This large sample of objects offers an unique opportunity to test conditions of fullerene formation and survival under different metallicity environments as we are analyzing five sources in our own Galaxy, four in the Large Magellanic Cloud, and seven in the Small Magellanic Cloud. Among the sixteen PNe under study, we present the first detection of C 60 (possibly also C 70 ) fullerenes in the PN M 1-60 as well as of the unusual ∼6.6, 9.8, and 20 µm features (attributed to possible planar C 24 ) in the PN K 3-54. Although selection effects in the original samples of PNe observed with Spitzer may play a potentially significant role in the statistics, we find that the detection rate of fullerenes in C-rich PNe increases with decreasing metallicity (∼5% in the Galaxy, ∼20% in the LMC, and ∼44 % in the SMC) and

Diffuse interstellar bands in fullerene planetary nebulae: the fullerenes – diffuse interstellar bands connection

“…Furthermore, the apparent lack of the strongest electronic transitions of the C 60 molecule in Tc 1 may indicate that we are not seeing emission from isolated, free C 60 molecules, which would explain the variable properties of the mid-IR C 60 spectral features observed in fullerene PNe (Bernard-Salas et al 2012;García-Hernández et al 2012a). At present, the HAC's photochemical processing is the most likely C 60 formation route in the complex circumstellar envelopes of PNe (García-Hernández et al 2012a;Bernard-Salas et al 2012;Micelotta et al 2012).…”

“…However, we cannot discard that the line of sight to Tc 1 may not intersect the fullerene-rich regions of the circumstellar shell (e.g., if the fullerenes may be formed in clumps). On the other hand, one can speculate that the strongest 3980 Å C 60 band -perhaps the other electronic bands, too -could be suppresed if the fullerenes are in the solid-state phase (e.g., Evans et al 2012;García-Hernández et al 2012a). However, a laboratory spectrum of C 60 dissolved in n-hexane displays the same transitions seen in the gas-phase (Cataldo, priv.…”

“…At present, the HAC's photochemical processing is the most likely C 60 formation route in the complex circumstellar envelopes of PNe (García-Hernández et al 2012a;Bernard-Salas et al 2012;Micelotta et al 2012). Larger fullerenes may grow from pre-existing C 60 molecules (Dunk et al 2012) that may be supplied by the photochemical processing of HAC dust, opening the possibility of forming other fullerene-based molecules such as buckyonions and fullerene adducts.…”

“…However, the presence of fullerenes in astrophysical environments has been a matter of debate until recently when Spitzer observations have provided evidence for C 60 and C 70 fullerenes from planetary nebulae (PNe; Cami et al 2010;García-Hernández et al 2010, 2011a, 2012a, reflection nebulae (Sellgren et al 2010) and the least H-deficient R Coronae Borealis (RCB) Appendix A is available in electronic form at http://www.aanda.org stars (García-Hernández et al 2011b,c). None of these environments is highly hydrogen-deficient.…”

“…None of these environments is highly hydrogen-deficient. Furthermore, the recent detection of C 60 fullerenes in PNe with normal H-abundances (García-Hernández et al 2010, 2011a, 2012a suggests that large fullerenes may be formed as decomposition products of hydrogenated amorphous carbon (HAC) dust and that fullerenes may be not so exotic and can form under conditions that are common to essentially all solar-like stars at the end of their lives.…”