Detection of unidentified molecular pure rotational lines in carbon-rich planetary nebulae. I. The fullerene-containing PN IC 418

Molecular emission is observed in a wide variety of astrophysical environments, yet a substantial fraction of spectral features detected at millimeter wavelengths remain unidentified. Identifying these features is essential for constraining the inventory of interstellar and circumstellar molecules and for understanding the chemical pathways operating in evolved stars, including those leading to fullerene formation in planetary nebulae (PNe). In this study, we investigate a set of weak unidentified molecular features detected in the carbon-rich and fullerene-containing PN We aim to constrain the nature of their molecular carriers and assess their possible connection to fullerene-related chemistry. High-sensitivity observations at 2, 3, and 7 were carried out using the RT40m and IRAM 30m radio telescopes. The spectral features found in these data sets were compared with public molecular spectroscopic databases. Spectral-pattern searches and line fits were performed under linear, symmetric, and asymmetric rotor approximations to estimate rotational constants of potential molecular carriers. We report the detection of 20 weak (SNR ∼ 2 - 13) unidentified features (UFs), none of which could be attributed to mm radio recombination lines, instrumental artifacts, or any known molecular species. The observed features are inconsistent with the regular line spacing expected from linear molecules or symmetric rotors. A recurrent doublet-like pattern was identified, although no other spectral patterns were found. The estimated rotational constant (B ∼ 2,500-3,660 suggests these are carriers with 4-13 atoms. The rotational constants estimated for the spectral lines are incompatible with those expected from C_60 derivatives (B ∼ 100 disfavoring fullerene-related species as the dominant carriers of the detected UFs. The detected UFs point to the presence of molecular species that have not yet been identified in space within the fullerene-rich PN Non-planar carbonaceous molecules produced during the destruction and processing of hydrogenated amorphous carbon (HAC) grains are suggested as promising candidates. The catalog of UFs presented here has been made publicly available to facilitate future comparisons with laboratory measurements and theoretical calculations, which will be essential for identifying molecular carriers and understanding the chemical pathways leading to fullerene formation in PNe.