Letter to the Editor

A study of C₄H₃N isomers in TMC-1: Line by line detection of HCCCH₂CN^⋆

¹ Grupo de Astrofísica Molecular, Instituto de Física Fundamental, CSIC, C/ Serrano 123, 28006 Madrid, Spain
e-mail: nuria.marcelino@csic.es
² Observatorio Astronómico Nacional (IGN), C/ Alfonso XII 3, 28014 Madrid, Spain
³ Observatorio de Yebes (IGN), Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain

Received: 19 December 2020
Accepted: 20 January 2021

Abstract

We present Yebes 40 m telescope observations of the three most stable C₄H₃N isomers towards the cyanopolyyne peak of TMC-1. We have detected 13 transitions from CH₃C₃N (A and E species), 16 lines from CH₂CCHCN, and 27 lines (a-type and b-type) from HCCCH₂CN. We thus provide a robust confirmation of the detection of HCCCH₂CN and CH₂CCHCN in space. We have constructed rotational diagrams for the three species and obtained rotational temperatures between 4 and 8 K as well as similar column densities for the three isomers, in the range (1.5−3) × 10¹² cm⁻². Our chemical model provides abundances of the order of the observed ones, although it overestimates the abundance of CH₃CCCN and underestimates that of HCCCH₂CN. The similarity of the observed abundances of the three isomers suggests a common origin, most probably involving reactions of the radical CN with the unsaturated hydrocarbons methyl acetylene and allene. Studies of reaction kinetics at low temperatures and further observations of these molecules in different astronomical sources are needed to draw a clear picture of the chemistry of C₄H₃N isomers in space.