Letter to the Editor

Discovery of fulvenallene in TMC-1 with the QUIJOTE line survey^⋆

¹ Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: jose.cernicharo@csic.es
² Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 96822, USA
³ Observatorio de Yebes, IGN, Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain
⁴ Observatorio Astronómico Nacional, IGN, C/Alfonso XII 3, 28014 Madrid, Spain

Received: 1 July 2022
Accepted: 11 July 2022

Abstract

We report the detection of fulvenallene (c-C₅H₄CCH₂) in the direction of TMC-1 with the QUIJOTE¹ line survey. Thirty rotational transitions with K_a = 0,1,2,3 and J = 9−15 were detected. The best rotational temperature fitting of the data is 9 K and a derived column density is (2.7 ± 0.3) × 10¹² cm⁻², which is only a factor of 4.4 below that of its potential precursor cyclopentadiene (c-C₅H₆), and 1.4–1.9 times higher than that of the ethynyl derivatives of cyclopentadiene. We searched for fulvene (c-C₅H₄CH₂), a CH₂ derivative of cyclopentadiene, for which we derive a 3σ upper limit to its column density of (3.5 ± 0.5) × 10¹² cm⁻². Upper limits were also obtained for toluene (C₆H₅CH₃) and styrene (C₆H₅C₂H₃), the methyl and vinyl derivatives of benzene. Fulvenallene and ethynyl cyclopentadiene are likely formed in the reaction between cyclopentadiene (c-C₅H₆) and the ehtynyl radical (CCH). However, the bottom-up gas-phase synthesis of cycles in TMC-1 underestimates the abundance of cyclopentadiene by two orders of magnitude, which strengthens the need to study all possible chemical pathways to cyclisation in cold dark cloud environments, such as TMC-1. However, the inclusion of the reaction between C₃H₃⁺ and C₂H₄ produces a good agreement between model and observed abundances.