In 1985, several unidentified lines observed toward several sources, were assigned to the aromatic molecule cyclopropenylidene by
P. Thaddeus, J. M. Vrtilek, and C. A. Gottlieb,
Laboratory and Astronomical Identification of Cyclopropenylidene, C<sub>3</sub>H<sub>2</sub>,
Astrophys. J. 299, L63–L66 (1985).

The first observation of an initially unidentified line concerned one of the ortho ground state lines 212 – 101 in emission at 82093.54 GHz toward the diverse molecular clouds Ori A, Sgr B2(OH), and TMC-1 by
P. Thaddeus, M. Guélin, and R. A. Linke,
Three New 'Nonterrestrial' Molecules,
Astrophys. J. 246, L41–L45 (1981).

An interesting aspect has been found by
H. E. Matthews, S. C. Madden, L. W. Avery, and W. M. Irvine,
The C<sub>3</sub>H<sub>2</sub> 2<sub>20</sub> – 2<sub>11</sub> Transition: Absorption in Cold Dark Clouds,
Astrophys. J. 307, L69–L73 (1986).

Observations of the two 13C isotopomers of cyclopropenylidene were reported first by J. Gómez-González, M. Guélin, J. Cernicharo, C. Kahane, and M. Bogey,
Detection of Interstellar <sup>13</sup>C Isotopes of C<sub>3</sub>H<sub>2</sub>,
Astron. Astrophys. 168, L11–L12 (1986).
They observed the 212 – 101 transitions toward Sgr B2(OH).

M. B. Bell, P. A. Feldman, H. E. Matthews, and L. W. Avery reported on the
Detection of Deuterated Cyclopropenylidene (C<sub>3</sub>HD) in TMC-1,
Astrophys. J. 311, L89–L92 (1986).
The detection of multiply substituted isotopologs is currently hampered by the lack of published laboratory data.

In 2012, laboratory data were published on several isotopologs, including the doubly deuterated one and the ones with one D and one 13C. However, data are still missing on the isotopomers with two 13C. It does not come as a surprise that
S. Spezzano, S. Brünken, P. Schilke, P. Caselli, K. M. Menten, M. C. McCarthy, L. Bizzocchi, S. P. Treviño-Morales, Y. Aikawa, and S. Schlemmer
reported on the
Interstellar Detection of //c//-C<sub>3</sub>D<sub>2</sub>,
Astrophys. J. 769, Art. No. L19 (2013)
in the dark clouds TMC-1C and L1544 using the IRAM 30 m radio telescope. Three transitions were detected in the 3 mm wavelength region.

M. Agúndez, N. Marcelino, J. Cernicharo, E. Roueff, and M. Tafalla
reported on
A Sensitive //λ// 3 mm Line Survey of L483 A Broad View of the Chemical Composition of a Core around a Class 0 object,
Astron. Astrophys. 625, Art. No. A147 (2019)
Among the many species observed is cyclopropenylidene with one D and one 13C.

Cyclopropenylidene was also detected in the circumstellar envelope of the carbon-rich AGB star CW Leo by
K. Kawaguchi, Y. Kasai, S.-I. Ishikawa, N. Kaifu,
A Spectral-Line Survey Observation of IRC +10216 between 28 and 50 GHz,
Publ. Astron. Soc. Japan 47, 853–876 (1995).

The molecule was also detected in the circumstellar envelope of a carbon-rich post-AGB star by
P. Cox, R. Güsten, and C. Henkel,
Detection of the Hydrocarbon Ring Molecule C<sub>3</sub>H<sub>2</sub> in the Planetary Nebula NGC 7027,
Astron. Astrophys. 181, L19–L22 (1987).

P. Cox, R. Güsten, and C. Henkel reported on the
Observations of C<sub>3</sub>H<sub>2</sub> in the Diffuse Interstellar Medium,
Astron. Astrophys. 206, 108–116 (1988).

Contributor(s): H. S. P. Müller; 03, 2011; 05, 2013; 07,2019

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