On the detections of C60 and derivatives in circumstellar environments

Abstract

C 60 (buckminsterfullerene) was recently discovered in a variety of circumstellar environments by the Spitzer Space Telescope, suggesting that the envelopes around evolved stars are active sites for the synthesis of fullerenes. However, the physical state, excitation mechanism, and formation route of circumstellar C 60 are not completely understood so far. These open issues are discussed in this paper. For that purpose we investigate the observed wavelengths and strengths of C 60 bands and compare them with the experimental values. We also statistically study the environments and emission properties of the C 60 sources. We would like to stress that improved flux measurements and more accurate Einstein coefficients are required to draw solid conclusions. Furthermore, we present possible detections of hydrogenated C 60 and C + 60 , and discuss their implications on fullerene chemistry in circumstellar environments.

A search for hydrogenated fullerenes in fullerene-containing planetary nebulae

“…Of note, fulleranes may also be by-products of this conversion from HAC to fullerenes (Duley & Hu 2012). Interestingly, Zhang & Kwok (2013) have (tentatively) detected fulleranes in the proto-PN IRAS 01005+7910; three strong C-H stretching bands at 3.48, 3.51, and 3.58 µm are apparently present in its ISO spectrum with fluxes that are comparable to the one of the 3.3 µm feature. By assuming that all features have the same oscillator strength, they conclude that the calculated relative strengths indicate that the m value (degree of hydrogenation of the fulleranes C 60 H m ) may lie within the range from 25−40, which is consistent with the production of C 60 H 36 (the dominant product of the hydrogenation reaction of C 60 ; e.g., Cataldo & Iglesias-Groth 2009) 11 .…”

“…3). 9 Note that Zhang & Kwok (2013) Finally, we divided these fluxes by the area (∼0.6 arcsec 2 for both PNe) of the emission in the 3−4 µm range covered by our ISAAC observations.…”

“…Fulleranes could be formed via the Duley & Hu (2012) HAC de-hydrogenation process, mentioned above, in which fulleranes may be by-products from the conversion from PFs to C 60 , or they could be formed by the reaction of preexisting fullerenes (e.g., formed by the photochemical processing of HACs; Scott et al 1997) with atomic hydrogen. During 11 Zhang & Kwok (2013) suggest that C 60 H 18 might be also present. the short (∼10 2 -10 4 yr) transition phase, AGB-PN, H 2 12 may be photodissociated into atomic H owing to the gradual increase of the UV radiation field with the evolution of the central star and/or by shocks (H 2 dissociated through collisions) as a result of the fast post-AGB stellar winds.…”

“…Indeed, Zhang & Kwok (2013) have tentatively detected fulleranes in the Infrared Space Observatory (ISO) spectrum of the proto-PN IRAS 01005+7910, where three emission peaks at ∼3.48, 3.51, and 3.58 µm in the C-H stretching region seem to be present.…”

Organic Compounds in Circumstellar and Interstellar Environments

“…Although Tc-1 does not show UIE bands, the aliphatic 8 and 12 µm plateau features are strongly present (Figure 3). In the pre-planetary nebula IRAS 01005+7910, all three plateau features at 8, 12, and 17 µm seen in planetary nebulae with UIE bands are found (Zhang and Kwok 2013). As a matter of fact, plateau features are present in all planetary nebulae with C 60 (Otsuka et al 2013).…”