Formation of buckminsterfullerene (C ₆₀ ) in interstellar space

Abstract

Buckminsterfullerene (C 60 ) was recently confirmed as the largest molecule identified in space. However, it remains unclear how and where this molecule is formed. It is generally believed that C 60 is formed from the buildup of small carbonaceous compounds in the hot and dense envelopes of evolved stars. Analyzing infrared observations, obtained by Spitzer and Herschel, we found that C 60 is efficiently formed in the tenuous and cold environment of an interstellar cloud illuminated by strong ultraviolet (UV) radiation fields. This implies that another formation pathway, efficient at low densities, must exist. Based on recent laboratory and theoretical studies, we argue that polycyclic aromatic hydrocarbons are converted into graphene, and subsequently C 60 , under UV irradiation from massive stars. This shows that alternative-top-downroutes are key to understanding the organic inventory in space.fullerene | interstellar medium T he midinfrared spectra of a variety of astrophysical objects are dominated by band emission (strongest at 3.3, 6.2, 7.7, 8.6, and 11.2 μm) attributed to carbonaceous macromolecules [i.e., polycyclic aromatic hydrocarbons (PAHs)] (1). These molecules are large (30-100 C atoms), abundant (approximately 5% of the elemental carbon), and their ionization plays a key role in the energy balance of gas in the interstellar medium (ISM) and in protoplanetary disk. In addition to PAH bands, infrared signatures observed at 7.0, 8.5, 17.4, and 19.0 μm have been reported recently (2, 3) and found to coincide precisely with the emission of buckminsterfullerene (C 60 ) (4), a cage-like carbon molecule. This detection heralds the presence of a rich organic inventory and chemistry in space. However, observed abundances of C 60 challenge the standard ion-molecule or grain-surface chemistry formation routes, which build up molecules from small to large in the ISM. For that reason, it has been suggested that C 60 is formed in the hot and dense envelopes of evolved stars (5-7) in processes similar to those found in sooty environments (8-11), and eventually, is ejected in space. Yet, this scenario faces the problem that it has a limited efficiency (6). PAHs and C 60 are known to coexist in the ISM (3); however, so far, the connection between PAHs and C 60 -and in particular the possibility to go from one compound to the other in space-has not been investigated. In this paper, we present a study of PAH and C 60 chemical evolution in the NGC 7023 nebula, using Spitzer (12) and Herschel (13) infrared observations. Observational ResultsInfrared Observations of the NGC 7023 Nebula. Earlier Spitzer observations of the NGC 7023 reflection nebula have revealed a chemical evolution of PAHs: Deep in the cloud, emission is dominated by PAH clusters, which evaporate into free-flying PAHs when exposed to the UV radiation from the star (14-16). There, gaseous PAHs are, in turn, ionized. While the neutral PAHs are dominated by zig-zag edges-as demonstrated by the strong C-H solo out-of-plane modes-the ions have an armchair ...

Graphene etching on SiC grains as a path to interstellar polycyclic aromatic hydrocarbons formation

“…This situation leaves small patches of graphene weakly bound to a H-saturated substrate, ready to be ejected by thermal or photoexcitation processes. These molecules could be the origin of the recently proposed top-down interstellar carbon chemistry 29 and the reason why SiC is abundant in evolved stars but normally gone in the ISM 30 . Figure 3a,b shows some regions of the surface that have been heavily etched.…”

Polycyclic aromatic hydrocarbon ionization as a tracer of gas flows through protoplanetary disk gaps

“…As the unimolecular dissociation rate depends exponentially on the internal excitation temperature and, therefore, in a given radiation field, on the size of the PAH, PAHs with a size smaller than a critical size are fully stripped of all their H (and do not contribute emission in any C-H mode), while PAHs larger than this size are fully hydrogenated. Small fully dehydrogenated PAHs are quickly destroyed through C 2 loss (or isomerized to cages and fullerenes) by further UV photon absorption (Joblin 2003;Zhen et al 2014) and this is expected to be the dominant photo destruction route for small PAHs (Berné & Tielens 2012;Montillaud et al 2013). Hence, as small dehydrogenated PAHs are quickly destroyed, the I 6.2 /I 11.3 µm band ratio would show little variation.…”

“…The hydrogen coverage on a PAH is a balance between dehydrogenation of highly excited PAHs following UV absorption and hydrogenation reactions with atomic hydrogen (Tielens et al 1987;Le Page et al 2003). Theoretically and experimentally, it has been well established that dehydrogenation is an "on/off" switch; for example, when dehydrogenation becomes important, a PAH quickly loses all its hydrogens; over a small range of G 0 /n H (Ekern et al 1997;Tielens 2005;Berné & Tielens 2012;Zhen et al 2014). Hence, for any PAH, there is a critical G 0 /n H ratio and a PAH becomes fully hydrogenated (completely dehydrogenated) if the G 0 /n H ratio is less (larger) than this ratio.…”

Interstellar C60+

“…Thus, we obtain that the ionization fraction of C 60 is 38%. If we consider a maximum abundance of C 60 in NGC 7023 of 1.7 × 10 −4 of the elemental carbon (Berné & Tielens 2012), this implies an abundance of ∼1.0 × 10 −4 of the elemental carbon abundance for C + 60 . This is a factor at least 10 lower than the value derived by Foing & Ehrenfreund (1994) for the diffuse interstellar medium, assuming that C + 60 is the carrier of the two DIBs at 9577 and 9632 Å.…”

“…In the NGC 7023 reflection nebula, Sellgren et al (2010) have shown that C 60 is predominantly found in the regions closest to the star. In that part of the nebula, UV irradiation is high (above 10 4 times the interstellar standard radiation field), and PAH molecules are ionized (Rapacioli et al 2005;Berné et al 2007;Pilleri et al 2012), if not destroyed (Berné & Tielens 2012;Montillaud et al 2012). One could therefore expect C + 60 to be present in these regions.…”