J. Cernicharo, M. Agúndez, C. Cabezas, B. Tercero, N. Marcelino, J. R. Pardo, and P. de Vicente
reported on
Pure hydrocarbon cycles in TMC-1: Discovery of ethynylcyclopropenylidene, cyclopentadiene, and indene
Astron. Astrophys. 649, Art. No. L15 (2021).
Indene, c-C₉H₈, was identified as the first pure polycyclic aromatic hydrocarbon through a large number of mostly a-type lines with 10 ≤ J ≤ 20 and K_a ≤ 6 in the course of a molecular line survey of the prototypical cold dark molecular cloud TMC-1 carried out with the Yebes 40 m radio telescope between 32.8 and 49.2 GHz. A substantial fraction of the lines was unblended or slightly blended. A value of T_rot = 10 K was assumed. The column density of indene is about the same as cyclopentadienene or the C₄H₄ isomers butenyne and ethynylallene. It is about a factor of 7 lower than that of C₃H₃.

A. M. Burkhardt, K. L. K. Lee, P. B. Changala, C. N. Shingledecker, I. R. Cooke, R. A. Loomis, H. Wei, S. B. Charnley, E. Herbst, M. C. McCarthy, and B. A. McGuire
submitted a report somewhat earlier but that appeared later on the
Discovery of the pure polycyclic aromatic hydrocarbon indene (//c//-C₉H₈) with GOTHAM observations of TMC-1
Astrophys. J. Lett. 913, Art. No. L18 (2021).
The observations were obtained in the course of the line survey Green Bank Telescope Observations of TMC-1: Hunting for Aromatic Molecules. The data were recorded with very high resolution which resolves, at least in part, the complex velocity structure . No individual lines were detected, as usually in this survey. Stacking of all lines of all four major velocity components, however, yielded a signal slighly above 5σ. Note, however, that this method becomes questionable with weak lines as those of indene. As can be seen in Fig. 4 of the previous paper, TMC-1 is not line poor anymore at 1 cm wavelength on the level of the indene lines; the situation may be slightly better at longer wavelengths. The derived column densities of the two reports agree.

Contributor(s): H. S. P. Müller; 05, 2021