On the presence of metallofullerenes in fullerene-rich circumstellar envelopes

TL;DR

This study models metallofullerenes' IR spectra to explain observed variations in fullerene IR bands in circumstellar environments, suggesting they are significant contributors and can be detected by JWST.

Contribution

The paper introduces quantum-chemistry based IR spectral simulations of metallofullerenes, proposing them as key contributors to fullerene IR features in space environments.

Findings

Metallofullerenes significantly contribute to IR bands attributed to C60.

Band ratio variations depend on metal content and ionization levels.

JWST can potentially detect metallofullerenes in space.

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 17.4um/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.