Letter to the Editor

Discovery of interstellar phenalene (c-C₁₃H₁₀): A new piece in the chemical puzzle of PAHs in space

¹ Departamento de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
² Departamento de Química Física y Química Inorgánica, Facultad de Ciencias-I.U. CINQUIMA, Universidad de Valladolid, 47011 Valladolid, Spain
³ Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS) and Departamento de Química Orgánica, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain
⁴ Observatorio Astronómico Nacional (OAN, IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain
⁵ Observatorio de Yebes, IGN, Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain

^⋆ Corresponding authors: carlos.cabezas@csic.es, j.cernicharo@csic.es

Received: 31 July 2025
Accepted: 19 August 2025

Abstract

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.