Detection of Buckminsterfullerene emission in the diffuse interstellar medium

Abstract

Emission of fullerenes in their infrared vibrational bands has been detected in space near hot stars. The proposed attribution of the diffuse interstellar bands at 9577 and 9632 Å to electronic transitions of the buckminsterfullerene cation (i.e. [Formula: see text]) was recently supported by new laboratory data, confirming the presence of this species in the diffuse interstellar medium (ISM). In this letter, we present the detection, also in the diffuse ISM, of the 17.4 and 18.9 μm emission bands commonly attributed to vibrational bands of neutral C₆₀. According to classical models that compute the charge state of large molecules in space, C₆₀ is expected to be mostly neutral in the diffuse ISM. This is in agreement with the abundances of diffuse C₆₀ we derive here from observations. We also find that C₆₀ is less abundant in the diffuse ISM than in star-forming regions, supporting the theory that C₆₀ can be formed in these regions.

Keywords: ISM : lines and bands; ISM: molecules; infrared : ISM.

Fig. 1

Upper panels : Images of the Chamaeleon (translucent) and Scutum (diffuse) lines of sight studied in this article, observed with Spitzer-IRAC (Werner et al. 2004a; Fazio et al. 2004). Positions of the spectroscopic observations are indicated with white crosses. The images show IRAC colours red=8.0μm, green=4.5μm, and blue=3.6μm. Inserts show the HI and CO emission spectra (McClure-Griffiths et al. 2009; Dame et al. 2001; Boulanger et al. 1998). Lower panels: Mid-infrared spectra towards the studied lines of sight are represented. The vertical lines indicate the positions of C₆₀ (red) and ${\text{C}}_{60}^{+}$ (black) bands observed in the NGC 7023 reflection nebula (Sellgren et al. 2010; Berné et al. 2013). The band at 7.0 μm observed in the diffuse line of sight is due to [ArII].

Fig. 2

Mid-infrared spectra extracted over the 15-20 μm range for both lines of sight (see Appendix A for details). The spectrum of the NGC 7023 nebula where C₆₀ emission has been reported (Sellgren et al. 2010) is shown as a reference. The Chamaeleon spectrum is compared with a low resolution spectrum (IRS-LL) of NGC 7023, while the Scutum line of sight spectrum is compared to a medium resolution spectrum (IRS-SH) of NGC 7023, to be consistent with the observing modes used to obtain the spectra of interest here (see Appendix A). The positions of the C₆₀ vibrational bands are indicated with red vertical patches. The 16.4 μm attributed to PAH emission is indicated with a grey patch. Spectrally unresolved lines of molecular hydrogen and ions are also indicated with vertical dotted lines.