2021A&A...649L..15C

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 3.1x10¹¹cm^–2 for the first cycle and similar values, in the range (1-2)x10¹³cm^–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 (Agundez et al., 2021A&A...647L..10A; Cernicharo et al., 2021A&A...647L...2C; Cernicharo et al., 2021A&A...647L...3C), 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.