Previous searches

Kroto (1985) proposed that the fullerene species (C₆₀ and related species) play an important role in interstellar physics and chemistry. This argument is based on the stability of the ``buckyball'' structure of fullerene compounds, as observed in the laboratory and confirmed by ab-initio calculations. The formation mechanisms invoked to produce such spherical carbonaceous structures are usually based on reactions in circumstellar shells, with precursors such as acetylene and corannulene (Goeres and Seldmayr 1992; Kroto and Jura 1992; Bettens et al. 1997). Another way of obtaining fullerenes, together with polycyclic aromatic hydrocarbons (PAHs), is the photoerosion of larger hydrogenated amorphous carbon grains (Scott et al. 1997). Fullerene or fullerane species have been searched for at various wavelengths in interstellar spectra. They have also been proposed as explanations for many unidentified interstellar features: the extinction bump at 2175 Å, the diffuse interstellar bands, the infrared emission features, and the extended red emission (Webster 1993a,b,c, 1996).

The C₆₀ absorption spectrum shows an absorption band in the ultraviolet (UV) range, at 3860 Å (Heath et al. 1987). Snow and Seab (1989) and Somerville and Bellis (1989) have searched for this feature without any clear evidence of its presence. They derived a C₆₀ upper limit of the order of 0.01% of the cosmic carbon abundance in the diffuse medium, and approximately 0.7% of the carbon abundance in Mira stars.

More recently, Clayton et al. (1995) have searched for the infrared (IR) emission features of C₆₀ at 8.6 $\mu$m in the spectra of carbon stars. No feature was seen in RCB stars at a level of 2% of the continuum. According to these authors, only the evolved star IRC+10216 possibly shows a 8.6 $\mu$m band that may be assigned to C₆₀.

The search for fullerene and fullerane species in the interstellar spectra is up to now characterised by a lack of laboratory data to identify the spectral features, except for rare species in reduced wavelength ranges. The main improvements should come in the coming years from the growth of experimental data.

The dominant ionization state of C₆₀ in the interstellar environment has been modeled (Bakes and Tielens 1995; Salama et al. 1996; Dartois and d'Hendecourt 1997), and showed that it would be mostly neutral and anionic in the diffuse medium, and cationic in more irradiated clouds such as reflection nebulae near a B star.

Laboratory work by Petrie et al. (1993) has shown that C₆₀⁺ is the most stable species among 60 atom fullerenes, because of its ``exceptional unreactivity'' with other interstellar material.

The laboratory spectrum of C₆₀⁺ shows two electronic bands at 9583 and 9645 Å (10435 and 10368 cm^-1) in a Neon matrix (Fulara et al. 1993). The corresponding absorption bands have been searched for in the interstellar medium, taking into account the wavelength shift induced by the matrix polarisability. The two new interstellar bands observed at 9577 and 9632 Å (10442 and 10382 cm^-1) by Foing and Ehrenfreund (1994, 1997) have been tentatively assigned to the cation fullerene C₆₀⁺. To deduce the abundance from the observations requires an estimation of the oscillator strength. The most reliable estimates (f = 0.003-0.006 from Fulara et al. 1993) lead to a total abundance of 0.6 to 1.2% of cosmic carbon in this species alone (Moutou et al. 1996b).

More recent observations of the 9577 and 9632 Å DIBs (Jenniskens et al. 1997) lead to a lower value of the measured equivalent width, for a different estimate of the telluric contribution, and thus to a lower abundance range for C₆₀⁺: 0.18 to 0.37% of cosmic carbon.