Letter to the Editor

Pure hydrocarbon cycles in TMC-1: Discovery of ethynyl cyclopropenylidene, cyclopentadiene, and indene^⋆

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

Received: 22 April 2021
Accepted: 28 April 2021

Abstract

We report the detection for the first time in space of three new pure hydrocarbon cycles in TMC-1: c-C₃HCCH (ethynyl cyclopropenylidene), c-C₅H₆ (cyclopentadiene), and c-C₉H₈ (indene). We derive a column density of 3.1 × 10¹¹ cm⁻² for the first cycle and similar values, in the range (1−2) × 10¹³ cm⁻², for the second and third. This means that cyclopentadiene and indene, in spite of their large size, are exceptionally abundant, only a factor of five less abundant than the ubiquitous cyclic hydrocarbon c-C₃H₂. The high abundance found for these two hydrocarbon cycles together with the high abundance previously found for the propargyl radical (CH₂CCH) and other hydrocarbons, such as vinyl and allenyl acetylene (Agúndez et al. 2021, A&A, 647, L10; Cernicharo et al. 2021a, A&A, 647, L2; Cernicharo et al. 2021b, A&A, 647, L3), start to allow us to quantify the abundant content of hydrocarbon rings in cold dark clouds and to identify the intermediate species that are probably behind the in situ bottom-up synthesis of aromatic cycles in these environments. While c-C₃HCCH is most likely formed through the reaction between the radical CCH and c-C₃H₂, the high observed abundances of cyclopentadiene and indene are difficult to explain through currently proposed chemical mechanisms. Further studies are needed to identify how five- and six-membered rings are formed under the cold conditions of a cloud such as TMC-1.