Discovery of interstellar phenalene ($c$-C$_{13}$H$_{10}$): A new piece for the chemical puzzle of PAHs in space

TL;DR

This paper reports the first interstellar detection of phenalene, a polycyclic aromatic hydrocarbon, in TMC-1, using radio astronomy, quantum calculations, and laboratory spectroscopy to confirm its presence and quantify its abundance.

Contribution

The discovery of phenalene in space is novel, combining astronomical observations, quantum chemistry, and laboratory experiments to identify a new PAH in the interstellar medium.

Findings

Detected 267 rotational transitions of phenalene in TMC-1.

Confirmed identification through laboratory microwave spectroscopy.

Estimated phenalene column density as (2.8±1.6)×10^13 cm^-2.

Abstract

We present the discovery of the unsubstituted polycyclic aromatic hydrocarbon (PAH) phenalene ($c$-C$_{13}$H$_{10}$) in TMC-1 as part of the QUIJOTE line survey. In spite of the low dipole moment of this three-ring PAH we have found a total of 267 rotational transitions with quantum numbers $J$ and $K_a$ up to 34 and 14, respectively, corresponding to 100 independent frequencies. The identification of this new PAH from our survey was based on the agreement between the rotational parameters derived from the analysis of the lines and those obtained by quantum chemical calculations. Subsequent chemical synthesis of this PAH and the investigation of its laboratory microwave spectrum unequivocally support our identification. The column density of phenalene in TMC-1 is (2.8$\pm$1.6)$\times$10$^{13}$ cm$^{-2}$.