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

We present Yebes 40m 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 4-8 K and 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 temperature 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.