On the Presence of Metallofullerenes in Fullerene-rich Circumstellar Envelopes

Abstract

The presence of neutral C _60 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 C _60 17.4 μ m/18.9 μ m 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 C _60 . These simulations may explain the large range of 17.4 μ m/18.9 μ m 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 μ m/18.9 μ m 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.

Keywords

• Astrochemistry
• Circumstellar matter
• Infrared spectroscopy
• Planetary nebulae
• R Coronae Borealis variable stars