2024A&A...690L..13C

We report the discovery in TMC-1 of two cyano derivatives of the PAH acenaphthylene (C₁₂H₈). We have found two series of lines with the QUIJOTE line survey that we assign to 1-C₁₂H₇CN and 5-C₁₂H₇CN. For the 1-isomer, we have detected and assigned 173 rotational transitions with J up to 46 and K_a up to 9, corresponding to 107 independent frequencies. For the 5-isomer, the identification is based on 56 individual lines, corresponding to 117 rotational transitions with J up to 40 and K_a up to 8. Identification of the carriers was achieved through a careful analysis of the derived rotational constants, which permit us to focus on molecules larger than naphthalene but smaller than anthracene and phenanthrene. Moreover, the derived rotational constants indicate that the species are planar; this allows us to discard derivatives of fluorene and acenaphthene, which are non-planar species. Quantum chemical calculations and subsequent chemical synthesis of these molecules, as well as the observation of their rotational transitions in the laboratory, unequivocally support our identifications. We also confirm, via a robust line-by-line detection, the previous claimed detection of 1- and 2-cyanonaphthalene, which were obtained through statistical stacking techniques. The column densities of 1- and 5-cyanoacenaphthylene are (9.5 ± 0.9) × 10¹¹ cm^–2, while those of 1- and 2-cyanonapthalene are (5.5 ± 0.5) × 10¹¹ cm^–2. Hence, it seems that acenaphthylene could be a factor of 1.7 more abundant than naphthalene. These results support a scenario in which PAHs grow in cold dark clouds based on fused five- and six-membered carbon rings.