Bernard-Salas, J. ; Cami, J. ; Peeters, E. ; Jones, A. P. ; Micelotta, E. ; Groenewegen, M. A.

published in American Astronomical Society Meeting Abstracts #221, 221, pp. 330.03 (2013)

Abstract: Recently, we reported the first detection of fullerenes (C60, C70) in space, specifically in Tc1, a young planetary nebulae (PNe). Once injected into the ISM, these stable species survive and are thus probably widespread in the Galaxy where they contribute to interstellar extinction, heating processes, and complex chemical reactions. Following our discovery, fullerenes have now been detected in a wide range of sources (post-AGBs, PNe, reflection nebula, HII regions, H-poor stars, and YSOs), showing that when conditions are favorable, fullerenes are formed in large quantities. The challenge resides now in determining its excitation mechanisms (fluorescence vs. thermal), as this sets their diagnostic value. To this end we have analyzed the mid-IR spectrum of three fullerene-rich PNe Tc1, SMP SMC 16, and SMP LMC 56 and compared them with thermal and fluorescence models. The strength of the fullerene bands in the three planetary nebulae is very similar, while the strength of the radiation field, is markedly different. Additionally, the spatial profile of different dust components in Tc1 indicates that the fullerene emission (C60) peaks far away from the central source. All this is hard to reconcile with a thermal origin for the fullerene excitation, and thus favors fluorescence as the excitation mechanism. In addition, the spectra in the three PNe show a strong broad plateau with substructure in the 6-9um range, and which is likely related to the fullerene formation mechanism. We present model spectra for 3 nm radius HAC particles and show that these can reproduce the 6-9um plateau, which may imply that fullerenes are formed by photo-chemical processing of hydrogenated amorphous carbon (HAC) nano-particles.

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