2025A&A...701L...8C

We present the discovery of the unsubstituted polycyclic aromatic hydrocarbon (PAH) phenalene (C₁₃H₁₀) in TMC-1 as part of the QUIJOTE line survey. In spite of the low dipole moment of this three-ring PAH, we managed to identify a total of 267 rotational transitions with quantum numbers J and K_a up to 34 and 14, respectively, corresponding to 71 independent frequencies. The identification of this new PAH from our survey was based on the agreement between the rotational parameters derived from the analysis of the lines and those obtained by quantum chemical calculations. Our subsequent chemical synthesis of this PAH and investigation of its laboratory microwave spectrum unequivocally support our identification. We report the column density of phenalene in TMC-1 as (2.8 ± 1.6) × 10¹³ cm^–2.