On the Thus Far Questionable Detection of Fullerenes in Extragalactic Sources



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 Galactic detection of C60 and C70, they appeared to be almost ubiquitous. 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). Some of 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 autors 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 in the Galactic section. However, some of the other detection are probably likely correct, even if some caution is advised.

D. A. García-Hernández, A. Manchado, P. García-Lario, L. Stanghellini, E. Villaver, R. A. Shaw, R. Szczerba, and J. V. Perea-Calderón,
studied with Spitzer
Formation of Fullerenes in H-containing Planetary Nebulae,
Astrophys. J. 724, Art. No. L39 (2010).
Besides several Galactic sources, they also studied LHA 115-N 42, also known as SMP SMC 16, in the Small Magellanic Cloud. Strong features attributed to C60 were detected near 17.4 and 18.9 μm. These features overlap with weaker features of C70 in the Galactic PN Tc 1. However, a C70 feature near 14.8 μm is clearly absent. The 8.5 μm feature of C60 was also observed weakly, the one near 7.0 μm is overlapped by a much stronger Ar+ line – similar to the situation in Tc 1. Because of the investigation mentioned above, this identification should be viewed with caution, though it appears to be rather likely.




C70

There are no reports on Extragalactic observations of C70 thus far.


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