Diffuse interstellar bands in fullerene planetary nebulae: the fullerenes – diffuse interstellar bands connection^⋆

¹ Instituto de Astrofísica de Canarias, C/ via Láctea s/n, 38200 La Laguna, Spain
e-mail: agarcia@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Spain

Received: 14 December 2012
Accepted: 1 January 2013

Abstract

We present high-resolution (R ~ 15   000) 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 4428 Å DIB and the weaker 6309 Å DIB are found to be unusually intense in Tc 1. We also report the detection of a new broad (FWHM ~ 5 Å) unidentified band at  ~6525 Å. We propose that the 4428 Å DIB (probably also the 6309 Å DIB and the new 6525 Å 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.