A study of CHN isomers in TMC-1: line by line detection of HCCCHCN.

We present Yebes 40m telescope observations of the three most stable CHN isomers towards the cyanopolyyne peak of TMC-1. We have detected 13 transitions from CHCN (A and E species), 16 lines from CHCCHCN, and 27 lines (a-type and b-type) from HCCCHCN. We thus provide a robust confirmation of the detection of HCCCHCN and CHCCHCN 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)×10 cm. Our chemical model provides abundances of the order of the observed ones, although it overestimates the abundance of CHCCCN and underestimates that of HCCCHCN. 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 CHN isomers in space.