===== On the Detection of Propynylidyne, l-C3H, in Space =====

C<sub>3</sub>H exists in two isomeric forms, a linear one and a cyclic one. The latter is considerably higher in energy. Still, both seem to be equally abundant in cold dark clouds. //l//-C<sub>3</sub>H was detected first toward the dense cold molecular cloud TMC-1 and in the circumstellar envelope of the carbon-rich AGB star CW Leo, aka IRC +10216, by\\
P. Thaddeus, C. A. Gottlieb, Å. Hjalmarson, L. E. B. Johansson, W. M. Irvine, P. Friberg, and R. A. Linke,\\
**[[https://ui.adsabs.harvard.edu/abs/1985ApJ...294L..49T|Astronomical Identification of the C<sub>3</sub>H Radical]]**,\\
//Astrophys. J.// **294**, L49–L53 (1985).\\
Four hfs components of the //J// = 3/2 – 1/2 transition near 32.6 GHz in the lower lying <sup>2</sup>Π<sub>1/2</sub> fine structure ladder were detected with the Onsala 20 m telescope. 2 parity split lines each were detected with several telescopes toward CW Leo for the 7/2 – 5/2, 9/2 – 7/2, and 13/2 – 11/2 transitions of the <sup>2</sup>Π<sub>1/2</sub> ladder near 76.2, 98.0, and 141.7 GHz as well as the 9/2 – 7/2 transitions of the <sup>2</sup>Π<sub>3/2</sub> ladder near 103.3 GHz.

The radical was also observed in three translucent molecular clouds CB 17, CB 24, and CB 228 by\\
B. E. Turner, E. Herbst, and R. Terzieva,\\
**[[https://doi.org/10.1086/313301|The Physics and Chemistry of Small Translucent Molecular Clouds. XIII. The Basic Hydrocarbon Chemistry]]**,\\
//Astrophys. J. Suppl. Ser.// **126**, 427–460 (2000).

The higher lying (CCH) bending mode is subjected to very strong Renner-Teller interaction causing one of the Σ components to be only 27.2 cm<sup>–1</sup> (39.1 K) above ground. Transitions within this state were observed in the circumstellar envelope of the carbon rich late type star CW Leo by\\
J.Cernicharo, M. Guélin, and C. Kahane,\\
**[[https://doi.org/10.1051/aas:2000147|A λ 2 mm Molecular Line Survey of the C-star Envelope IRC+10216]]**,\\
//Astron. Astrophys. Suppl. Ser.// **142**, 181–215 (2000).

Propynylidyne was also detected in the circumstellar envelope of the post-AGB object (protoplanetary nebula) V353 Aur by\\
J. R. Pardo and J. Cernicharo,\\
**[[https://ui.adsabs.harvard.edu/abs/2007ApJ...654..978P|Molecular Abundances in CRL 618]]**,\\
//Astrophys. J.// **654**, 978–987 (2007).


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Contributor(s): H. S. P. Müller; 09, 2011

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