Pure hydrocarbon cycles in TMC-1: Discovery of ethynyl
  cyclopropenylidene, cyclopentadiene and indene
  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 3.1e11
cm-2 for the former cycle and similar values, in the range (1-2)e13 cm-2, for
the two latter molecules. 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 like
vinyl 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 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 are five- and six-membered rings formed
under the cold conditions of clouds like TMC-1.
