Letter to the Editor

Discovery of 1H-cyclopent[cd]indene (c-C₁₁H₈) in TMC-1 with the QUIJOTE line survey: A new three-ringed polycyclic aromatic hydrocarbon^★

¹ Dept. de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC) C/Serrano 121 28006 Madrid, Spain
² Observatorio de Yebes (IGN), Cerro de la Palera s/n 19141 Yebes Guadalajara, Spain
³ Observatorio Astronómico Nacional (OAN, IGN), C/Alfonso XII 3 28014 Madrid, Spain

^★★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 December 2025
Accepted: 31 December 2025

Abstract

We report the detection of the polycyclic aromatic hydrocarbon (PAH) 1H-cyclopent[cd]indene (c-C₁₁H₈) in TMC-1 with the QUIJOTE line survey. We detected 22 independent lines corresponding to 88 rotational transitions with quantum numbers ranging from J = 19 up to J = 24 and K_a ≤ 5 in the Q-band range. The identification of this new PAH was based on the agreement between the rotational parameters derived from the analysis of the lines and those obtained by quantum chemical calculations. The column density derived for 1H-cyclopent[cd]indene is (6.0 ± 0.5) × 10¹² cm⁻², with a rotational temperature of 9 K. Its abundance is high, as is that of the rest of the PAHs, but it is the lowest of all those detected to date in TMC-1, being 2.66 times less abundant than indene and 4.66 times less than phenalene. This result will help us to better understand the growth of five- and six-membered rings in dark clouds. Chemical models explaining their formation through the bottom-up model are still very incomplete and require further experimental and theoretical effort. Even so, the most likely formation reactions would occur between the smallest rings with small hydrocarbons; the most probable reaction for the formation of cyclopentindene is that between indene and C₂H, C₂H₃, and/or their cation.