In the last decade, fullerenes have been detected in a variety of astrophysical environments, with the majority being found in planetary nebulae. Laboratory experiments have provided us with insights into the conditions and pathways that can lead to fullerene formation, but it is not clear precisely what led to the formation of astrophysical fullerenes in planetary nebulae. We review some of the available evidence, and propose a mechanism where fullerene formation in planetary nebulae is the result of a two-step process where carbonaceous dust is first formed under unusual conditions; then, the fullerenes form when this dust is being destroyed

Bernard Salas, Jeronimo

Cami, Jan

De Buizer, James

Doppmann, Greg

Peeters, Els

Galaxies

English

Jan Cami

Multidisciplinary Digital Publishing Institute

The Formation of Fullerenes in Planetary Nebulae

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe Formation of Fullerenes in Planetary NebulaeJournal ItemHow to cite:Cami, Jan; Peeters, Els; Bernard Salas, Jeronimo; Doppmann, Greg and De Buizer, James (2018). The Formation ofFullerenes in Planetary Nebulae. Galaxies, 6(4), article no. 101.For guidance on citations see FAQs.c© [not recorded]Version: Version of RecordLink(s) to article on publisher’s website:http://dx.doi.org/doi:10.3390/galaxies6040101Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukgalaxiesArticleThe Formation of Fullerenes in Planetary NebulaeJan Cami 1,2,* , Els Peeters 1,2 , Jeronimo Bernard-Salas 3,4 , Greg Doppmann 5 andJames De Buizer 61 Department of Physics and Astronomy and Centre for Planetary Science and Exploration (CPSX),The University of Western Ontario, London, ON N6A 3K7, Canada; epeeters@uwo.ca2 SETI Institute, 189 Bernardo Ave, Suite 100, Mountain View, CA 94043, USA3 Robert Hooke Building, Department of Physical Sciences, The Open University,Milton Keynes MK7 6AA, UK; Jeronimo.Bernard-Salas@open.ac.uk4 ACRI-ST, 260 Route du Pin Montard, 06904 Sophia-Antipolis, France5 W. M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, HI 96743, USA;gdoppmann@keck.hawaii.edu6 Stratospheric Observatory for Infrared Astronomy-USRA, NASA Ames Research Center, MS N232-12,Moffett Field, CA 94035, USA; jdebuizer@sofia.usra.edu* Correspondence: jcami@uwo.ca; Tel.: +1-519-661-2111 (ext. 80978)Received: 31 July 2018; Accepted: 18 September 2018; Published: 21 September 2018Abstract: In the last decade, fullerenes have been detected in a variety of astrophysical environments,with the majority being found in planetary nebulae. Laboratory experiments have provided us withinsights into the conditions and pathways that can lead to fullerene formation, but it is not clearprecisely what led to the formation of astrophysical fullerenes in planetary nebulae. We review someof the available evidence, and propose a mechanism where fullerene formation in planetary nebulaeis the result of a two-step process where carbonaceous dust is first formed under unusual conditions;then, the fullerenes form when this dust is being destroyed.Keywords: planetary nebulae; fullerenes1. IntroductionWhen Kroto et al. [1] conducted a series of experiments to simulate the chemistry occurring in thesurroundings of carbon-rich evolved stars, they discovered a new and particularly stable carbonaceousmolecule: Buckminsterfullerene, C60. We now know that C60 is the most stable (and best known)member of an entire class of large, cage-like carbonaceous molecules. Given the stability of themolecule and the nature of the simulation experiments, Kroto et al. [1] immediately concluded that C60was most likely widespread and abundant in space, and as soon as spectroscopic data were available,astronomers searched for its telltale signature in interstellar and circumstellar environments see [2].We can now confirm that C60 is indeed widespread and abundant in space. Since the firstunambiguous detection of all IR active vibrational modes of C60 in the Spitzer-IRS spectrum ofthe planetary nebula (PN) Tc 1 at 7.0, 8.5, 17.4, and 18.9 µm [3], the same spectral features havebeen found in a variety of evolved star environments (see Section 3), as well as in ReflectionNebulae RNe; see e.g., [4,5], the diffuse ISM [6], and young stellar objects and Herbig Ae/Be stars [7].Recently, laboratory experiments and astronomical observations have confirmed the identification oftwo strong (and 3 weaker) diffuse interstellar bands DIBs; see [8] as due to electronic transitions ofthe C+60 cation see e.g., [9–12], and references therein. It is estimated that, on average, C60 locks up10−4–10−3 of the cosmic carbon [9,13,14], a considerable abundance for a single species!While it is thus established that C60 is abundantly present in interstellar and circumstellarenvironments, the life cycle of the C60 molecule itself is not crystal clear. Here, we review what weGalaxies 2018, 6, 101; doi:10.3390/galaxies6040101 www.mdpi.com/journal/galaxiesGalaxies 2018, 6, 101 2 of 6know about the conditions that lead to the formation of fullerenes and how this can be reconciled withthe body of available observational evidence to construct a coherent formation mechanism for fullerenesin evolved star environments.2. The Conditions Required to form FullerenesLaboratory experiments (as well as theoretical calculations) on the condensation of carbonaceousgas have provided important information about the carbon chemistry that can occur in circumstellarenvironments. The key parameter that determines the outcome is the temperature (see Figure 1 fora schematic overview). At low temperatures, the condensation products are a variety of (small)molecules (e.g., C2H2, HCN...); any dust that condenses out will be primarily in the form ofamorphous carbon [15]. Above ∼1000 K, the nature of the possible chemical reactions changes,and now benzene can form through a series of chemical reactions and from this species, a wholefamily of polycyclic aromatic hydrocarbon (PAH) molecules [16,17]. These PAHs are then thecondensation nuclei for the formation of soot; the soot is formed quickly, and is graphitic in nature [17].Finally, similar experiments at much higher temperatures (above ∼3500 K) result in the formation offullerene molecules in addition to a soot that is fullerenic in nature [17]. Thus, different temperaturesresult in distinctly different products: no fullerenes are formed at the PAH-favoured temperatures,and similarly no PAHs appear in the high temperature experiments. This is partly due to hydrogenatoms that inhibit the formation of fullerenes at the lower temperatures; when similar experiments arecarried out in H-poor conditions, fullerenes do form at these lower temperatures [18].1000 K<T<1700 KT<1000 K T& 3500 KH-poor?Aliphatics +amorphouscarbonPAHs +graphiticsootC60 +fullerenicsootnoyesUV UVFigure 1. Formation routes for carbonaceous materials based on [15–18].Irradiation of these carbonaceous compounds by UV photons can then significantly changetheir nature. It has long been known that UV irradiation of amorphous carbon leads to aromatizationin the compounds. More recently, theoretical calculations [13,19] and laboratory experiments [20]have shown that large PAHs are first stripped of their H atoms by UV photons; the resulting puregraphene flake then curls up and shrinks by emission of C2 units until it reaches the most stableconfiguration—C60. It is thus conceivable that a significant amount of carbonaceous dust can betransformed by these processes over their lifetime—from aliphatic to aromatic to fullerenic material.3. Observations of Fullerenes in PNeWhile the IR C60 bands have been detected in various environments, the majority of detectionsoriginates from evolved stars (including post-AGB and pre-PN objects, but primarily in PNe in theMilky Way and the Magellanic Clouds; see e.g., [21–23] and references therein). All C60-PNe havevery similar properties and IR spectra. Indeed, they are all fairly young, low-excitation objects, the IRspectra of which are characterized by a rising, featureless continuum on which several emissionfeatures are superposed: The C60 bands, a strong 11–13 µm emission plateau (possibly due toGalaxies 2018, 6, 101 3 of 6SiC), a 6–9 µm emission plateau, and a strong 30 µm feature [22,23]. Thus, the conditions thatare conducive to the formation and excitation of C60 also result in the appearance of other dustcomponents. Notably, PAH emission in these objects is very weak or absent [23]. When comparingoptical and near-IR colors of these objects to other evolved stars, the fullerene sources are the leastreddened objects (i.e., the direct lines of sight to the central stars do not contain much dust that cancause reddening) and cluster together in near-IR color-color diagrams, while other evolved objectshave a large range in colors [24].Spatial studies of the different emission components can give clues to the conditions and originof the different components. Narrow-band images of Tc 1 clearly show different distributions ofthe ionized gas, the dust responsible for the continuum emission, and the fullerenes (see Figure 2).They reveal that the fullerenes emit in a ring ∼ 5′′ away from the central star, at the interface betweenthe ionized region and the surrounding Photo-Dissociation Region (PDR). There is no clear evidencefor PAHs in this object, but if a very weak 11.3 µm bump is to be associated with PAHs, then thoseare all located in the top part of the central horse-shoe shaped region associated with the continuumdust emission. In that case, the PAHs and fullerenes would clearly be spatially separated, with thefullerenes at a location where the radiation field should be weaker than at the location of the PAHs.Figure 2. Decomposition of Gemini/T-ReCS narrow-band infrared images of Tc 1 into the differentcomponents (Cami et al., in prep): the continuum dust emission (red), the ionized gas (blue), and thefullerene emission (green).Finally, an important observational result is that the C60 detection rate in PNe is fairly low:Only 3–5% of the Galactic PNe observed by Spitzer-IRS at high resolution exhibit the C60 bands [23];the fraction of C60 detections in the Magellanic Clouds is slightly higher [22]. These low rates implythat the C60 molecules either form in a small fraction of objects (possibly related to requiring specificformation and/or excitation conditions), or alternatively that pure C60 has only a short lifetimein these environments, possibly related to chemically altering the molecular structure that wouldsignificantly affect the spectroscopic signatures.4. DiscussionThe circumstellar environments of evolved stars represent what happened to an atomic gas(the stellar envelope) that cooled after ejection due to mass loss processes. When considering theformation of molecules and dust in these environments, we thus always start from bottom up processes.In most cases (for typical condensation temperatures), we would expect the end products in C-richGalaxies 2018, 6, 101 4 of 6environments to be PAHs and graphitic dust. Only when the temperatures are high or the environmentis H-poor would fullerenes and fullerenic dust result.The most straightforward scenario for the formation of fullerenes is thus one where the C60-PNerepresent objects where—for some reason—the condensation happens at higher temperatures or underH-poor conditions. This must have happened close to the central star (otherwise temperatures anddensities would be too low), and consequently the condensation products moved outward since itsformation—at least for Tc 1 where we see the C60 emission far from the central star. Since the formationof C60 also results in large amounts of fullerenic soot, while no dust is seen at the location of theC60 emission, dust destruction must have occurred as well. This would then explain why the colors ofthe C60-PNe are so similar and appear unreddened. Perhaps the C60 molecules are the only speciesto survive this process, but it is conceivable that the destruction of fullerenic dust actually results inthe formation of C60 molecules, e.g., by “liberating” C60 molecules from the dust grains. This dustdestruction process must then be related to the transition of a post-AGB object to a PN since we onlysee the C60 bands in young PNe. Shock waves related to a fast wind overtaking a slow wind, or thedevelopment of an ionization front appear to be the most likely direct causes. The formation of C60in PNe would thus involve a two-step process: Condensation (in an earlier phase, likely the AGB)under unusual conditions to produce C60 and fullerenic dust, followed by the destruction of the dust(that possibly creates more C60). Such a scenario can explain why the C60-PNe have the observationalproperties they have.There is of course a fundamentally different formation route for C60 that we should also consider.Given that the central stars of PNe are hot stars that produce copious amounts of UV photons,photo-processing of carbonaceous materials seems like a plausible way to produce fullerenes,and indeed, it has been suggested that the formation of C60 is the result of photo-processing ofhydrogenated amorphous carbon (HAC) dust grains or similar carbon clusters or PAHs see e.g., [21,25].However, if this process can efficiently convert carbonaceous dust to fullerenes in the low-excitationC60-PNe, the same process should be even more efficient in the more mature objects where timescalesfor processing have been longer, and UV photons are more abundant. Similarly, much of the dustin Tc 1 would have been converted to fullerenes as well, since it is located five times closer to thesource of UV photons than the fullerenes themselves. In other words, if photo-processing of HACs(or even PAHs) would be the general process for fullerene formation in young PNe, there should onlybe very few PNe left that have circumstellar PAHs or in fact any carbonaceous material at all otherthan fullerenes.The only remaining question is then why the dust condensation conditions were different inthe C60-PNe, and this may be tied to the evolutionary history of the central stars. For about half ofthe C60-PNe, it has been established that their central stars are of [WC] type see e.g., [23]; certainly forsuch objects, it is plausible that condensation could happen in H-poor environments that would bemore suited to create fullerenes and fullerenic soot than other objects. It is not clear what the statusof the other C60-PNe is in that sense, and we should also point out that not all PNe with [WC] starsdisplay the fullerenes. Future observations that allow to measure e.g., the carbon abundance would becrucial to determine the underlying cause for the different condensation conditions.Author Contributions: Conceptualization, J.C., E.P. and J.B.-S.; Data curation, J.D.B.; Formal analysis, J.C.;Funding acquisition, J.C., E.P. and G.D.; Methodology, J.C.; Software, J.C.; Visualization, J.C.; Writing—originaldraft, J.C.; Writing—review & editing, J.C., E.P., J.B.-S. and G.D.Funding: Natural Sciences and Engineering Research Council of Canada: RGPIN-2016-06047.Acknowledgments: J.C. and E.P. acknowledge support from an NSERC DG.Conflicts of Interest: The authors declare no conflict of interest.Galaxies 2018, 6, 101 5 of 6References1. Kroto, H.W.; Heath, J.R.; Obrien, S.C.; Curl, R.F.; Smalley, R.E. C60: Buckminsterfullerene. Nature 1985,318, 162–163. [CrossRef]2. Herbig, G.H. The Search for Interstellar C60. Astrophys. J. 2000, 542, 334–343. [CrossRef]3. Cami, J.; Bernard-Salas, J.; Peeters, E.; Malek, S.E. Detection of C60 and C70 in a Young Planetary Nebula.Science 2010, 329, 1180–1182. [CrossRef] [PubMed]4. Sellgren, K.; Werner, M.W.; Ingalls, J.G.; Smith, J.D.T.; Carleton, T.M.; Joblin, C. C60 in Reflection Nebulae.Astrophys. J. Lett. 2010, 722, L54–L57. [CrossRef]5. Peeters, E.; Tielens, A.G.G.M.; Allamandola, L.J.; Wolfire, M.G. The 15–20 µm Emission in the ReflectionNebula NGC 2023. Astrophys. J. 2012, 747, 44. [CrossRef]6. Berné, O.; Cox, N.L.J.; Mulas, G.; Joblin, C. Detection of buckminsterfullerene emission in the diffuseinterstellar medium. Astron. Astrophys. 2017, 605, L1. [CrossRef] [PubMed]7. Roberts, K.R.G.; Smith, K.T.; Sarre, P.J. Detection of C60 in embedded young stellar objects, a Herbig Ae/Bestar and an unusual post-asymptotic giant branch star. Mon. Not. R. Astron. Soc. 2012, 421, 3277–3285.[CrossRef]8. Foing, B.H.; Ehrenfreund, P. Detection of Two Interstellar Absorption Bands Coincident with SpectralFeatures of C+60. Nature 1994, 369, 296. [CrossRef]9. Campbell, E.K.; Holz, M.; Gerlich, D.; Maier, J.P. Laboratory confirmation of C+60 as the carrier of two diffuseinterstellar bands. Nature 2015, 523, 322–323. [CrossRef] [PubMed]10. Walker, G.A.H.; Bohlender, D.A.; Maier, J.P.; Campbell, E.K. Identification of More Interstellar C+60 Bands.Astrophys. J. Lett. 2015, 812, L8. [CrossRef]11. Walker, G.A.H.; Campbell, E.K.; Maier, J.P.; Bohlender, D. The 9577 and 9632 Å Diffuse Interstellar Bands:C+60 as Carrier. Astrophys. J. 2017, 843, 56. [CrossRef]12. Cordiner, M.A.; Cox, N.L.J.; Lallement, R.; Najarro, F.; Cami, J.; Gull, T.R.; Foing, B.H.; Linnartz, H.;Lindler, D.J.; Proffitt, C.R.; et al. Searching for Interstellar C+60 Using a New Method for High Signal-to-noiseHST/STIS Spectroscopy. Astrophys. J. Lett. 2017, 843, L2. [CrossRef]13. Berné, O.; Tielens, A.G.G.M. Formation of buckminsterfullerene (C60) in interstellar space. Proc. Natl. Acad.Sci. USA 2012, 109, 401–406. [CrossRef] [PubMed]14. Omont, A. Interstellar fullerene compounds and diffuse interstellar bands. Astron. Astrophys. 2016, 590, A52.[CrossRef]15. Gail, H.P.; Sedlmayr, E. Formation of crystalline and amorphous carbon grains. Astron. Astrophys. 1984,132, 163–167.16. Frenklach, M.; Feigelson, E.D. Formation of polycyclic aromatic hydrocarbons in circumstellar envelopes.Astrophys. J. 1989, 341, 372–384. [CrossRef]17. Jäger, C.; Huisken, F.; Mutschke, H.; Jansa, I.L.; Henning, T. Formation of Polycyclic Aromatic Hydrocarbonsand Carbonaceous Solids in Gas-Phase Condensation Experiments. Astrophys. J. 2009, 696, 706–712.[CrossRef]18. Wang, X.K.; Lin, X.W.; Mesleh, M.; Jarrold, M.F.; Dravid, V.P.; Ketterson, J.B.; Chang, R.P.H. The effect ofhydrogen on the formation of carbon nanotubes and fullerenes. J. Mater. Res. 1995, 10, 1977–1983. [CrossRef]19. Berné, O.; Montillaud, J.; Joblin, C. Top-down formation of fullerenes in the interstellar medium.Astron. Astrophys. 2015, 577, A133. [CrossRef] [PubMed]20. Zhen, J.; Castellanos, P.; Paardekooper, D.M.; Linnartz, H.; Tielens, A.G.G.M. Laboratory Formation ofFullerenes from PAHs: Top-down Interstellar Chemistry. Astrophys. J. Lett. 2014, 797, L30. [CrossRef]21. García-Hernández, D.A.; Manchado, A.; García-Lario, P.; Stanghellini, L.; Villaver, E.; Shaw, R.A.;Szczerba, R.; Perea-Calderón, J.V. Formation of Fullerenes in H-containing Planetary Nebulae. Astrophys. J.Lett. 2010, 724, L39–L43. [CrossRef]22. García-Hernández, D.A.; Iglesias-Groth, S.; Acosta-Pulido, J.A.; Manchado, A.; García-Lario, P.;Stanghellini, L.; Villaver, E.; Shaw, R.A.; Cataldo, F. The Formation of Fullerenes: Clues from New C60,C70, and (Possible) Planar C24 Detections in Magellanic Cloud Planetary Nebulae. Astrophys. J. Lett. 2011,737, L30. [CrossRef]23. Otsuka, M.; Kemper, F.; Cami, J.; Peeters, E.; Bernard-Salas, J. Physical properties of fullerene-containingGalactic planetary nebulae. Mon. Not. R. Astron. Soc. 2014, 437, 2577–2593. [CrossRef]Galaxies 2018, 6, 101 6 of 624. Sloan, G.C.; Lagadec, E.; Zijlstra, A.A.; Kraemer, K.E.; Weis, A.P.; Matsuura, M.; Volk, K.; Peeters, E.;Duley, W.W.; Cami, J.; et al. Carbon-rich Dust Past the Asymptotic Giant Branch: Aliphatics, Aromatics,and Fullerenes in the Magellanic Clouds. Astrophys. J. 2014, 791, 28. [CrossRef]25. Micelotta, E.R.; Jones, A.P.; Cami, J.; Peeters, E.; Bernard-Salas, J.; Fanchini, G. The Formation of CosmicFullerenes from Arophatic Clusters. Astrophys. J. 2012, 761, 35. [CrossRef]c© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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