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We report the detection for the first time in space of three new purehydrocarbon cycles in TMC-1: c-C3HCCH (ethynyl cyclopropenylidene), c-C5H6(cyclopentadiene) and c-C9H8 (indene). We derive a column density of 3.1e11cm-2 for the former cycle and similar values, in the range (1-2)e13 cm-2, forthe two latter molecules. This means that cyclopentadiene and indene, in spiteof their large size, are exceptionally abundant, only a factor of five lessabundant than the ubiquitous cyclic hydrocarbon c-C3H2. The high abundancefound for these two hydrocarbon cycles, together with the high abundancepreviously found for the propargyl radical (CH2CCH) and other hydrocarbons likevinyl and allenyl acetylene (Agundez et al. 2021; Cernicharo et al. 2021a,b),start to allow us to quantify the abundant content of hydrocarbon rings in colddark clouds and to identify the intermediate species that are probably behindthe in situ bottom-up synthesis of aromatic cycles in these environments. Whilec-C3HCCH is most likely formed through the reaction between the radical CCH andc-C3H2, the high observed abundances of cyclopentadiene and indene aredifficult to explain through currently proposed chemical mechanisms. Furtherstudies are needed to identify how are five- and six-membered rings formedunder the cold conditions of clouds like TMC-1.

Pure hydrocarbon cycles in TMC-1: Discovery of ethynyl cyclopropenylidene, cyclopentadiene and indene