  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 17.4um/18.9um band ratios indicate that either additional
emitters should contribute to the astronomical IR spectra or there exist
unknown physical processes 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 species 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 17.4um/18.9um 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 17.4um/18.9um band ratio in the metallofullerenes
simulated IR spectra mainly depends on the metal abundances, ionization level,
and endo/exo concentration 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 enviroments.
