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

TL;DR

This paper reports the first detection of three pure hydrocarbon cycles in space within TMC-1, revealing their high abundance and prompting new questions about their formation mechanisms in cold dark clouds.

Contribution

It introduces the first in-space detection of ethynyl cyclopropenylidene, cyclopentadiene, and indene, highlighting their unexpected abundance and challenging existing chemical formation models.

Findings

Cyclopentadiene and indene are only five times less abundant than common cyclic hydrocarbons.

High abundances suggest significant presence of hydrocarbon rings in cold dark clouds.

Current chemical models cannot fully explain the formation of these rings.

Abstract

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.