Diffuse interstellar bands in fullerene planetary nebulae: the fullerenes - diffuse interstellar bands connection

We present high-resolution (R similar to 15 000) VLT/UVES optical spectra of two planetary nebulae (PNe; Tc 1 and M 1-20) where C-60 (and C-70) fullerenes have already been found. These spectra are of high quality (S/N > 300) for PN Tc 1, which permits us to search for the expected electronic transitions of neutral C-60 and diffuse interstellar bands (DIBs). Surprisingly, we report the non-detection of the most intense optical bands of C-60 in Tc 1, although this could be explained by the low C-60 column density estimated from the C-60 infrared bands if the C-60 emission peaks far away from the central star. The strongest and most common DIBs in both fullerene PNe are normal for their reddening. Interestingly, the very broad 4428 angstrom DIB and the weaker 6309 angstrom DIB are found to be unusually intense in Tc 1. We also report the detection of a new broad (FWHM similar to 5 angstrom) unidentified band at similar to 6525 angstrom. We propose that the 4428 angstrom DIB (probably also the 6309 angstrom DIB and the new 6525 angstrom band) may be related to the presence of larger fullerenes (e. g., C-80, C-240, C-320, and C-540) and buckyonions (multishell fullerenes such as C-60@C-240 and C-60@C-240@C-540) in the circumstellar envelope of Tc 1.