A Small Fullerene (C24) may be the Carrier of the 11.2 micron Unidentified Infrared Band

Abstract

We analyze the 11.2 μm unidentified infrared band (UIR) spectrum from NGC 7027 and identify a small fullerene (C24) as a plausible carrier. The blurring effects of lifetime and vibrational anharmonicity broadening obscure the narrower, intrinsic spectral profiles of the UIR band carriers. We use a spectral deconvolution algorithm to remove the blurring, in order to retrieve the intrinsic profile of the UIR band. The shape of the intrinsic profile, a sharp blue peak and an extended red tail, suggests that the UIR band originates from a molecular vibration-rotation band with a blue band head. The fractional area of the band-head feature indicates a spheroidal molecule, implying a non-polar molecule and precluding rotational emission. Its rotational temperature should be well approximated by that measured for non-polar molecular hydrogen, ~825 K for NGC 7027. Using this temperature, and the inferred spherical symmetry, we perform a spectral fit to the intrinsic profile that results in a rotational constant implying C24 as the carrier. We show that the spectroscopic parameters derived for NGC 7027 are consistent with the 11.2 μm UIR bands observed for other objects. We present density functional theory (DFT) calculations for the frequencies and infrared intensities of C24. The DFT results are used to predict a spectral energy distribution (SED) originating from absorption of a 5 eV photon, and characterized by an effective vibrational temperature of 930 K. The C24 SED is consistent with the entire UIR spectrum and is the dominant contributor to the 11.2 and 12.7 μm bands.

arXiv comment20 pages, 8 figures, accepted for publication in ApJ, replaced incorrect figure 8

References in corpus (18)

• The rich 6 to 9 um spectrum of interstellar PAHs
• The profiles of the 3 to 12 $\mu$m PAH features
• On-line database of the spectral properties of polycyclic aromatic hydrocarbons
• Evolution of PAHs in photodissociation regions: Hydrogenation and charge states
• Laboratory formation of fullerenes from PAHs: Top-down interstellar chemistry
• Top-down formation of fullerenes in the interstellar medium
• Properties of the close binary and circumbinary torus of the Red Rectangle
• Neutral and charged excitations in carbon fullerenes from first-principles many-body theories
• ISO observations toward the reflection nebula NGC 7023: A nonequilibrium ortho- to para-H2 ratio
• A Spitzer Study of 21 and 30 Micron Emission in Several Galactic Carbon-rich Proto-Planetary Nebulae
• Rotationally Warm Molecular Hydrogen in the Orion Bar
• On the viability of the PAH model as an explanation of the unidentified infrared emission features
• The 11.2 $\mu$m emission of PAHs in astrophysical objects
• C${}_{60}$ in Photodissociation Regions
• On the origin of the 11.3 micron unidentified infrared emission feature
• On the Carriers of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars
• Interpreting the subtle spectral variations of the 11.2 and 12.7 μm polycyclic aromatic hydrocarbon bands
• C60 as a probe for astrophysical environments

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• The EDIBLES survey V: Line profile variations in the $\lambda \lambda$5797, 6379, and 6614 diffuse interstellar bands as a tool to constrain carrier sizes
• Unusual Infrared Emission toward Sgr B2: Possible Planar C${}_{24}$