2025ApJ...986...77R

We investigate large fullerenes containing between 90 and 1750 carbon atoms and with radii ranging from 4.4 to 12.9 Å, and compare their computed emission spectra to Spitzer data for the fullerene-rich planetary nebulae SMP SMC 24 and SMP LMC 48 to assess their contribution to the broad infrared emission plateaus. The structures are computed without any bias using molecular dynamics simulations and contain mostly five- and six-membered rings, several seven-membered rings, and very few four- and eight-membered rings. Our largest structures consist of multishell fullerenes. The infrared spectra are calculated using the density-functional-based tight-binding theory. Their emission spectra exhibit broad plateaus from 6 to 10 μm and from 14 to 20 μm, whose 6-10/14-20 relative intensity ratios reach a maximum value for ∼200 carbons. The computed 6-10 μm plateaus exhibit distinct subfeatures in agreement with the observations. The subfeatures with the largest intensities peak at 6.6 and 7.7-7.9 μm. We assign these subfeatures to C-C stretching vibrations from the different types of rings arranged in many different ways. A comparison of the computed 6-10/14-20 μm intensity ratios as a function of fullerene size and photon energy with the observed 6-10/14-20 μm intensity ratio for SMP LMC 48 allows us to derive an upper bound of ∼150-200 carbons for the fullerene size.