In search for the carriers of Diffuse Interstellar Bands (DIB), Foing & Ehrenfreund (1994) reported the discovery of two DIBs at 9577 and 9632 Å that are close to the laboratory position of two strong electronic transitions of C60, which were measured by matrix isolation spectroscopy in neon at 9580 4 and 9642 3 Å with an intensity ratio of 1:1.5 (Fulara et al. 1993). Fullerenes had been proposed as a possible candidate for the DIBs (Kroto 1987, Léger et al. 1988) and the identification was quickly embraced (e.g. Smith 1995) in spite of its far reaching implications (e.g. Herbig 1995), not in the least because there seemed to be only two such strong DIBs in a wide range of the infrared spectrum (Ehrenfreund et al. 1995, Foing & Ehrenfreund 1995). The implications were that C60 had to be a common molecule in the diffuse interstellar medium, where it should play an important role in gas phase chemistry and in the heating and cooling of the gas. Recently, it was shown that C60 might be formed in the gas phase through a chemical network involving ion-neutral reactions (Bettens & Herbst 1996), taking away earlier objections that a plausible ISM formation mechanism was lacking.
Questions were immediately raised. First of all concerning the observations, because the nature of the absorption features was not well established due to problems with the removal of water lines and the low detector and grating sensitivity at those near-infrared wavelengths. Observations from a dryer site were called for. Also, the Foing & Ehrenfreund (1994) paper did not report on the two weaker vibronic transitions that had been measured in the laboratory at 9366 and 9419 Å with a strength of about 29% and 15% of the two vibrationless transitions respectively. If observed, their equivalent width should scale to the reported bands in the 9600 range as in the matrix spectrum and should possess a comparable FWHM (Maier 1994).
Here, we report on a high spectral resolution study of the two "C60 bands" in the line of sight to three reddened stars with different stellar Doppler velocities. This allows us to discriminate between interstellar and stellar photospheric absorptions. We have also searched for the vibronic bands around 9400 Å. We confirm the existence of both DIBs, but find that the 9632 Å band relative to 9577 is weaker in dense cloud environments than previously reported.
© European Southern Observatory (ESO) 1997
Online publication: April 8, 1998