2013A&A...550L...6G

We present high-resolution (R ∼15000) VLT/UVES optical spectra of two planetary nebulae (PNe; Tc 1 and M 1-20) where C₆₀ (and C₇₀) 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 C₆₀ and diffuse interstellar bands (DIBs). Surprisingly, we report the non-detection of the most intense optical bands of C₆₀ in Tc 1, although this could be explained by the low C₆₀ column density estimated from the C₆₀ infrared bands if the C₆₀ 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 4428Å DIB and the weaker 6309Å DIB are found to be unusually intense in Tc 1. We also report the detection of a new broad (FWHM ∼5Å) unidentified band at ∼6525Å. We propose that the 4428Å DIB (probably also the 6309Å DIB and the new 6525Å band) may be related to the presence of larger fullerenes (e.g., C₈₀, C₂₄₀, C₃₂₀, and C₅₄₀) and buckyonions (multishell fullerenes such as C₆₀@C₂₄₀ and C₆₀@C₂₄₀@C₅₄₀) in the circumstellar envelope of Tc 1.