On the Detection of Fullerenes in Space



C60

Fullerenes, and in particular the most stable small fullerene C60, were sought for quite extensively. Initially, they appeared to be rather elusive. Shortly after the definitive detection of C60 and C70, described below in the C70 subsection, they appeared to be almost ubiquitous as described below and in the Extragalactic section. However, a very recent laboratory spectroscopic investigation by
W. W. Duley and A. Hu,
Fullerenes and Proto-fullerenes in Interstellar Carbon Dust,
Astrophys. J. 659, Art. No. L11 (2012);
concluded that the presence of PAH molecules and of fullerenes are mutually exclusive. In fact, the dehydrogenation of PAHs involves pseudo-fullerenes which still contain H atoms and which are not as symmetric as C60. Prior to turning to fullerenes, these molecules exhibit IR bands similar to C60, but in addition a strong mode near 16.4 μm (610 cm–1). The IR bands of fullerenes are apparently less specific than had been thought. This fact, however, had been known for the mid-infrared features of PAHs for a long time. As a consequence, the authors cast doubt on all reported detections of C60 described in this section and in the Extragalactic section. The only uncontested report appears to be the one in which C60 and C70 were detected in Tc 1, see below.

Probably the first, tentative report, which seemed to be substantiated later, was published by
M. W. Werner, K. I. Uchida, K. Sellgren, M. Marengo, K. D. Gordon, P. W. Morris, J. R. Houck, and J. A. Stansberry,
New Infrared Emission Features and Spectral Variations in NGC 7023,
Astrophys. J. Suppl. Ser. 154, 309–314 (2004).
The reflection nebula was studied with Spitzer. In a note added in proof, they mentioned that M. Jura of UCLA pointed out that features at 17.4 and about 19.0 μm may be assigned to C60.
A more recent and more detailed account by
K. Sellgren, K. I. Uchida, and M. W. Werner,
The 15–20 μm Spitzer Spectra of Interstellar Emission Features in NGC 7023,
Astrophys. J. 659, 1338–1351 (2007)
concluded that if the two modes are due to C60, then the 17.4 μm band is overlapped in certain spatial regions.
An even more recent paper by
K. Sellgren, M. W. Werner, J. G. Ingalls, J. D. T. Smith, T. M. Carleton, and C. Joblin,
C60 in Reflection Nebulae,
Astrophys. J. 722, L54–L57 (2010)
emphasizes that C60 should be present not only in NGC 7023, but also NGC 2023 because in addition to the two bands described above, a third of four fundamental vibrations seem to have been observed near 7.0 μm. The remaining band at 8.5 μm is strongly overlapped. The authors estimate that between 0.1 and 0.6 % of the interstellar carbon is locked up in C60.

Shortly before, however, a different article reported on the firm identification of C60 and C70 as described in the C70 section.

Three out of the four fundamental modes of C60 have reportedly been identified toward a star in the transition state from AGB star to planetary nebula by
Y. Zhang and S. Kwok,
Detection of C60 in the Protoplanetary Nebula IRAS 01005+7910,
Astrophys. J. 730, Art. No. 126 (2011).

C60 was suggested to be present in R Coronae Borealis (RCB) stars, which are very hydrogen-poor, but rich in He and C. In fact, it was found only in DY Cen and possibly V854 Cen, two comparatively H-rich RCB stars, by
D. A. García-Hernández, N. Kameswara Rao, D. L.Lambert,
Are C60 Molecules Detectable in Circumstellar Shells of R Coronae Borealis Stars?,
Astrophys. J. 729, Art. No. 126 (2011).




C70

J. Cami, J. Bernard-Salas, E. Peeters, and S. E. Malek,
reported on Detection of C60 and C70 in a Young Planetary Nebula,
Science 659, 1180–1182 (2010).
They have used archival Spitzer data of Tc 1. The C60 features at 17.4 and 18.9 μm overlap with slightly weaker C70 features at similar positions. The 8.5 μm feature of C60 was also observed, the one near 7.0 μm is overlapped by a much stronger Ar+ line. Weak C70 features at 12.5, 14.8, 15.6, and 21.8 μm seem to be free from overlap.
The authors point out that both C60 and C70 are unlikely to be in the gas phase. Rather, they reside on grains. An excitation temperature of 330 K was derived for C60 whereas 180  was obtained for C70, indicating the latter to occur farther out from the star.
Up to now, this article appears to be the only uncontested report on C60 and C70 because features commonly assigned to PAHs are absent in Tc 1.


Contributor(s): H. S. P. Müller; 10, 2010; 06, 2011; 01, 2012