2021A&A...655L...1C

We report the detection of two isomers of ethynyl cyclopentadiene (c-C₅H₅CCH), namely 1- and 2-ethynyl-1,3-cyclopentadiene, in the direction of TMC-1. We derive column densities of (1.4±0.2)x10¹²cm^–2 and (2.0±0.4)x10¹²cm^–2, respectively, for these two cyclopentadiene derivatives, which imply that they are about ten times less abundant than cyclopentadiene. We also report the tentative detection of ethynyl benzene (C₆H₅CCH), for which we estimate a column density of (2.5±0.4)x10¹²cm^–2. We derived abundances for the corresponding cyano derivatives of cyclopentadiene and benzene and found values significantly lower than previously reported. The rotational temperature of the ethynyl and cyano derivatives of these cycles is about 9 K, that is, very close to the gas kinetic temperature of the cloud. The abundance ratio of the 1- and 2-isomers of ethynyl cyclopentadiene is 1.4±0.5, while for the two isomers of cyano cyclopentadiene it is 2.4±0.6. The relative abundances of CCH over CN derivatives is 7.7±2.2 for cyclopentadiene, which probably reflects the abundance ratio of the radicals CCH and CN; this ratio is only 2.1±0.5 for benzene, which suggests that additional reactions besides cyano radicals with benzene are involved in the formation of benzonitrile. The formation of these cycles is reasonably well accounted for through a chemical scheme based on neutral-neutral reactions. It is predicted that benzene should be as abundant as cyclopentadiene in TMC-1.