On the Presence of Metallofullerenes in Fullerene-rich Circumstellar Envelopes

Abstract

The presence of neutral C60 fullerenes in circumstellar environments has been firmly established by astronomical observations as well as laboratory experiments and quantum-chemistry calculations. However, the large variations observed in the C60 17.4 μm/18.9 μm band ratios indicate that either additional emitters should contribute to the astronomical infrared (IR) spectra or unknown physical processes exist besides thermal and UV excitation. Fullerene-based molecules such as metallofullerenes and fullerene-adducts are natural candidate species as potential additional emitters, but no specific specie has been identified to date. Here we report a model based on quantum-chemistry calculations and IR spectra simulation of neutral and charged endo(exo)hedral metallofullerenes, showing that they have a significant contribution to the four strongest IR bands commonly attributed to neutral C60. These simulations may explain the large range of 17.4 μm/18.9 μm band ratios observed in very different fullerene-rich circumstellar environments like those around planetary nebulae and chemically peculiar R Coronae Borealis stars. Our proposed model also reveals that the 17.4 μm/18.9 μm band ratio in the metallofullerenes simulated IR spectra mainly depends on the metal abundances, ionization level, and endo/exoconcentration in the circumstellar envelopes. We conclude that metallofullerenes are potential emitters contributing to the observed IR spectra in fullerene-rich circumstellar envelopes. Our simulated IR spectra indicate also that the James Webb Space Telescope has the potential to confirm or refute the presence of metallofullerenes (or even other fullerene-based species) in circumstellar environments.

Infrared Spectral Fingerprint of Neutral and Charged Endo- and Exohedral Metallofullerenes

“…First, we consider the analysis of the individual mid-IR spectra for the different metallofullerenes, tracking the particular spectral features that define the metal-C 60 cage interaction for each case. To facilitate comparison, Figures 1-4 (representing all individual neutral and charged endo-and exohedral metallofullerenes spectra) also include the total (summed) spectrum for each metallofullerene species, constructed similarly to our previous work (Barzaga et al 2023). The IR intensities of the individual simulated spectra are scaled to the same total value.…”

“…Calculations of the neutral/charged endo-(inside C 60 ) and exohedral (outside C 60 ) metallofullerenes have been performed in the framework of density functional theory (DFT) at the B3LYP/ 6-31G(d) level (Ditchfield et al 1971;Becke 1993), with the Gaussian 16 code (Frisch et al 2016), following the settings reported in our previous work (Barzaga et al 2023). 9 The mid-IR spectra were obtained under the harmonic oscillator approximation, and subsequently, wavelengths were adjusted using double scaling factors to account for anharmonicity, vibrorotational coupling, and so on.…”

“…The individual IR spectra of the neutral endofullerene [M@C 60 ] are shown in Figure 1, highlighting the different metal species with different colors. There are two specific regions in these spectra that contain the four pristine C 60 IR features (see, e.g., Kern et al 2013;Barzaga et al 2023): (i) the 5-10 μm C-C stretching vibrations, and (ii) the 15-20 μm C 60 cage vibrations. The C-C stretching region shows a common trend for Li, Na, and K (alkaline metals) neutral endofullerenes.…”

“…In order to explore the possible existence of metallofullerenes in fullerene-rich circumstellar envelopes, we have previously performed quantum-chemistry calculations of the IR vibrational spectra of charged and neutral endo-and exohedral metallofullerenes (Barzaga et al 2023). According to the IR spectra simulation (i.e., the total and total-weighted metallofullerene spectra; see Barzaga et al 2023), the presence of metallofullerenes could explain the large range of C 60 17.4 μm/18.9 μm band ratios observed in PNe and RCB stars. Therefore, metallofullerenes can be considered as potential emitters that contribute to the observed IR spectra in the fullerene-rich circumstellar envelopes of these types of objects.…”

Towards a reliable prediction of the infrared spectra of cosmic fullerenes and their derivatives in the JWST era

“…Distorting the carbon cage by ionization or adsorption of foreign atoms or groups could reduce the symmetry and introduce more active infrared features. In fact, fullerenes undergo complicated chemical processing during stellar evolution, making the formation of hydrogenated fullerenes, fullerene-metal and fullerene-hydrocarbon complexes possible (Kroto & Jura 1992, Cami 2014, Zhang et al 2020, Hou et al 2023, Barzaga et al 2022, Omont 2016. Due to its high spectral resolution and unprecedented sensitivity, the advent of the James Webb Space Telescope (JWST) offers a valuable opportunity to detect fullerene derivatives through their vibrational bands.…”