Formation of fullerenes in H-containing Planetary Nebulae

Hydrogen depleted environments are considered an essential requirement for the formation of fullerenes. The recent detection of C60 and C70 fullerenes in what was interpreted as the hydrogen-poor inner region of a post-final helium shell flash Planetary Nebula (PN) seemed to confirm this picture. Here, we present evidence that challenges the current paradigm regarding fullerene formation, showing that it can take place in circumstellar environments containing hydrogen. We report the simultaneous detection of Polycyclic Aromatic Hydrocarbons (PAHs) and fullerenes towards C-rich and H-containing PNe belonging to environments with very different chemical histories such as our own Galaxy and the Small Magellanic Cloud. We suggest that PAHs and fullerenes may be formed by the photochemical processing of hydrogenated amorphous carbon. These observations suggest that modifications may be needed to our current understanding of the chemistry of large organic molecules as well as the chemical processing in space.

💡 Research Summary

The paper challenges the long‑standing view that fullerenes (C₆₀, C₇₀) can only be synthesized in hydrogen‑deficient astrophysical environments. Historically, the detection of fullerenes in the inner, supposedly H‑poor region of a planetary nebula (PN) that has undergone a final helium‑shell flash was taken as strong evidence for this paradigm. In contrast, the authors present a systematic observational study of carbon‑rich, hydrogen‑containing planetary nebulae both in our Galaxy and in the Small Magellanic Cloud (SMC), which have markedly different chemical histories. Using infrared spectroscopy, they simultaneously detect the characteristic emission bands of polycyclic aromatic hydrocarbons (PAHs) and the fullerene features of C₆₀ and C₇₀ in each target.

The coexistence of PAHs and fullerenes in the same circumstellar shells indicates that the two families of large carbon molecules are not mutually exclusive but can arise from a common precursor under the same physical conditions. The authors propose that hydrogenated amorphous carbon (HAC) grains, ubiquitous in carbon‑rich outflows, undergo photochemical processing when exposed to the intense ultraviolet (UV) radiation fields of the central stars. Laboratory analogues have shown that UV irradiation of HAC leads to dehydrogenation, structural rearrangement, and the formation of both aromatic ring systems (the building blocks of PAHs) and closed‑cage carbon clusters (the precursors of fullerenes). By comparing laboratory spectra with the astronomical data, the authors argue that the observed PAH and fullerene bands can be reproduced by a single HAC‑processing pathway.

Importantly, the phenomenon is observed in environments with very different metallicities: Galactic PNe, which typically have near‑solar metal abundances, and SMC PNe, which are metal‑poor. This suggests that metallicity, initial C/H ratios, or the detailed evolutionary history of the nebulae are secondary to the presence of UV photons and HAC material. The results therefore imply that a modest amount of hydrogen does not inhibit fullerene formation; rather, hydrogen can be incorporated into the precursor HAC and later expelled during the photochemical transformation.

The paper’s conclusions have two major implications for astrochemistry. First, the requirement of a hydrogen‑free environment for fullerene synthesis must be revised; fullerenes can form efficiently in H‑bearing circumstellar shells, provided that the UV field is strong enough to drive HAC processing. Second, PAHs and fullerenes should be considered as co‑products of the same photochemical evolution rather than as competing species. This calls for a re‑examination of existing models of large organic molecule chemistry in space, especially those that treat PAH and fullerene formation as independent pathways.

The authors outline future work that includes (i) systematic surveys of a larger sample of PNe across a range of UV fluxes and hydrogen abundances, (ii) laboratory experiments quantifying the efficiency of HAC conversion into PAHs versus fullerenes under controlled UV irradiation, and (iii) detailed modeling of the time‑dependent chemistry to predict the relative abundances of PAHs and fullerenes as a function of nebular age. Such studies will refine our understanding of carbon chemistry in late‑stage stellar evolution, inform models of dust processing in the interstellar medium, and potentially shed light on the origins of complex organic molecules in the early universe.