Letter to the Editor

Discovery of HCCCH₂CCH in TMC-1 with the QUIJOTE line survey^⋆

¹ Dept. de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: r.fuentetaja@csic.es; jose.cernicharo@csic.es
² Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
³ Observatorio Astronómico Nacional (OAN, IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain

Received: 28 November 2023
Accepted: 19 June 2024

Abstract

We present the first detection in space of 1,4-pentadiyne. It has been found towards TMC-1 with the QUIJOTE line survey in the 31–50 GHz range. We observed a total of 17 transitions with J = 2 up to 13 and K_a = 0, 1 and 2. The observed transitions allowed us to derive a rotational temperature of 9.5 ± 0.5 K and a column density of (5.0 ± 0.5) × 10¹² cm⁻². This molecule was the last non-cyclic isomer of the C₅H₄ family that could be detected via radio astronomy. A computational chemistry study was performed to determine the energies of the five most stable isomers. The isomer (c-C₃H₃CCH) has a considerably higher energy than the others, and it has not yet been detected. To better understand the chemical reactions involving these species, we compared the ethynyl and cyano derivatives. The observed abundances of these species are in good agreement with the branching ratios of the formation reactions studied with our chemical model of TMC-1.