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

Abstract

We present high-resolution (R ∼ 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 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., 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.

A search for diffuse bands in fullerene planetary nebulae: evidence of diffuse circumstellar bands

“…By using the Spitzer formula (N ∼ 10 20 × (EQW/(λ 2 × f ))), where f is the oscillator strength of each C 60 transition (Sassara et al 2001), this translates into column densities of ∼2 × 10 13 , 4 × 10 13 , and 2 × 10 13 cm −2 . These column density limits are similar to those previously obtained in Tc 1 by García-Hernández & Díaz-Luis (2013). By following the latter work, we could in principle compare these column-density limits with estimates of the circumstellar density of C 60 molecules as derived from the IR C 60 emission bands.…”

“…García-Hernández & Díaz-Luis (2013) found no evidence of these strong neutral C 60 optical bands in absorption (or emission) in the fullerene PN Tc 1. The S/N in the M 1-20 optical spectrum is too low to search for neutral C 60 features in its spectrum (García-Hernández & Díaz-Luis 2013), but here we have searched the higher S/N (∼100 in the continuum around 4000 Å) spectrum of the PN IC 418 for the strongest electronic transitions of neutral C 60 mentioned above.…”

“…However, from the O 2 telluric lines at ∼6970 Å, we measure a much higher resolving power of about 37 000. The signal-to-noise ratio (S/N) (in the final combined spectrum) in Tc 1 is very high (∼300 at 4000 Å and >300 at longer wavelengths), which permitted us to search for the expected electronic transitions of neutral C 60 and both strong and weak DIBs (García-Hernández & Díaz-Luis 2013). In M 1-20, however, the final S/N (∼20 at 4000 Å and >30 at wavelengths longer than 6000 Å) was not high enough to search the relatively broad (and weak) C 60 features around 4000 Å or the weakest DIBs.…”

Metallofullerene and fullerene formation from condensing carbon gas under conditions of stellar outflows and implication to stardust

“…Thus, reaction of PAHs with fullerenes provides additional insight into fundamental astrochemistry between these two important classes of large cosmic molecules. For example, a better understanding of the carbon structures that produce the enigmatic diffuse interstellar bands (DIBs) can also be achieved by such investigations (38). Very recently, it has been shown that small PAHs, such as anthracene, can form a DielsAlder cycloaddition product in the gas phase (39).…”

AN INFRARED DIFFUSE CIRCUMSTELLAR BAND? THE UNUSUAL 1.5272μm DIB IN THE RED SQUARE NEBULA

“…Furthermore, as circumstellar environments are typically significantly warmer and denser than diffuse ISM clouds, they provide additional, complementary regimes in which to characterize DIB behavior and thus their carriers. Searches for DIB features in circumstellar environments have been either unsuccessful or inconclusive in a variety of sources, from protostars to planetary nebulae (PNs; e.g., Le Bertre & Lequeux 1993;Oudmaijer et al 1997;García Hernández & Díaz-Luis 2013). Slightly more promising candidates have been found by Díaz-Luis et al (2015), who noted absorption features toward the PN Tc 1 at wavelengths consistent with DIBs blueshifted within the range of the nebular emission velocity; Dahlstrom et al (2013) and Oka et al (2013) interpreted some unusually broad and asymmetric DIBs toward the infrared (IR) source Herschel 36 as most likely due to excitation of the DIB carrier molecules by the local radiation field, but see also Bernstein et al (2015).…”