Discovery of 1H-cyclopent[cd]indene (c-C11H8) in TMC-1 with the QUIJOTE line survey: A new three-ringed polycyclic aromatic hydrocarbon

TL;DR

This paper reports the first detection of the PAH 1H-cyclopent[cd]indene in TMC-1, providing insights into PAH abundance and formation in dark clouds through radio line observations and quantum chemical analysis.

Contribution

The discovery of 1H-cyclopent[cd]indene in TMC-1 is the first identification of this specific PAH in space, expanding knowledge of complex organic molecules in dark clouds.

Findings

Detected 22 lines corresponding to 88 rotational transitions.

Column density of (6.0 +- 0.5) x 10^12 cm^-2 for 1H-cyclopent[cd]indene.

Abundance is lower than other PAHs like indene and phenalene.

Abstract

We report the detection of the polycyclic aromatic hydrocarbon (PAH) 1H-cyclopent[cd]indene (c-C11H8) in TMC-1 with the QUI- JOTE line survey. We detected 22 independent lines corresponding to 88 rotational transitions with quantum numbers ranging from J=19 up to J=24 and Ka <= 5 in the Q-band range. The identification of this new PAH was based on the agreement between the rotational parameters derived from the analysis of the lines and those obtained by quantum chemical calculations. The column density derived for 1H-cyclopent[cd]indene is (6.0 +- 0.5) x 10^12 cm-2, with a rotational temperature of 9 K. Its abundance is high, as is that of the rest of the PAHs, but it is the lowest of all those detected to date in TMC-1, being 2.66 times less abundant than indene and 4.66 times less than phenalene. This result will help us to better understand the growth of five- and six-membered rings in dark clouds. Chemical models explaining their formation through the bottom-up model are still very incomplete and require further experimental and theoretical effort. Even so, the most likely formation reactions would occur between the smallest rings with small hydrocarbons; the most probable reaction for the formation of cyclopentindene is that between indene and C2H, C2H3, and/or their cation.