Letter to the Editor

Cumulene carbenes in TMC-1: Astronomical discovery of l-H₂C₅^⋆

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

Received: 8 May 2021
Accepted: 31 May 2021

Abstract

We report the first detection in space of the cumulene carbon chain l-H₂C₅. A total of eleven rotational transitions, with J_up = 7−10 and K_a = 0 and 1, were detected in TMC-1 in the 31.0–50.4 GHz range using the Yebes 40 m radio telescope. We derived a column density of (1.8 ± 0.5)×10¹⁰ cm⁻². In addition, we report observations of other cumulene carbenes detected previously in TMC-1 in order to compare their abundances with the newly detected cumulene carbene chain. We find that l-H₂C₅ is ∼4.0 times less abundant than the larger cumulene carbene l-H₂C₆, while it is ∼300 and ∼500 times less abundant than the shorter chains l-H₂C₃ and l-H₂C₄. We discuss the most likely gas-phase chemical routes to these cumulenes in TMC-1 and stress that chemical kinetics studies able to distinguish between different isomers are needed to shed light on the chemistry of C_nH₂ isomers with n > 3.