The QUIJOTE ultrasensitive line survey has enabled the discovery of two cyano derivatives of the polycyclic aromatic hydrocarbon (PAH) acenaphthylene (C12H8) in the dark cloud TMC-1.
The first series of lines, labeled B429, corresponds to 1-cyanoacenaphthylene. A total of 173 rotational transitions with J up to 46 and Ka up to 9 have been detected and assigned, corresponding to 107 independent frequencies.
The second series, labeled B444, is assigned to 5-cyanoacenaphthylene. This identification is based on 56 individual lines, corresponding to 117 rotational transitions with J up to 40 and Ka up to 8.
The identification of the carriers was achieved through a careful analysis of the derived rotational constants, which indicate that the species are planar, allowing the authors to discard derivatives of non-planar molecules such as fluorene and acenaphthene. Quantum chemical calculations and subsequent chemical synthesis of these molecules, as well as the observation of their rotational transitions in the laboratory, unequivocally support the identifications.
The authors also confirm, through 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 estimated to be (9.5 ± 0.9) × 10^11 cm^-2, while those of 1- and 2-cyanonaphthalene are (5.5 ± 0.5) × 10^11 cm^-2. This suggests that acenaphthylene could be a factor of 1.7 more abundant than naphthalene in TMC-1.
These results support a scenario in which polycyclic aromatic hydrocarbons grow in cold dark clouds based on the formation of fused five- and six-membered carbon rings.
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