===== On the Detection of Ethynyl, C2H, in Space =====

The ethynyl radical in its <sup>2</sup>Σ ground vibrational state was detected with the NRAO 11 m telescope in the four stronger of the six hyperfine components of the //N// = 1 – 0 rotational transitions near 87.4 GHz by\\
K. D. Tucker, M. L. Kutner, and P. Thaddeus,\\
**[[https://ui.adsabs.harvard.edu/abs/1974ApJ...193L.115T|The Ethynyl Radical C<sub>2</sub>H – A New Interstellar Molecule]]**,\\
//Astrophys. J.// **193** L115–L119 (1974)\\
before it was actually detected in the laboratory. The identification was based on hyperfine parameters obtained from electron-spin resonance measurements of ethynyl isolated in argon matrices. The molecule is rather ubiquitous as it has been detected in several star-forming regions such as W3, Orion A, Sgr B2, Sgr A, of which several include photon-dominated regions (PDRs), most notably M17 SW. It has also been detected in the circumstellar envelope of the carbon-rich late-type star CW Leo, also known as IRC +10216.

A. Wootten, E. P. Bozyan, D. P. Garrett, R. B. Loren, and R. L. Snell reported on the\\
**[[https://ui.adsabs.harvard.edu/abs/1980ApJ...239..844W|Detection of C<sub>2</sub>H in Cold Dark Clouds]]**,\\
//Astrophys. J.// **239** 944–854 (1980).

The radical was also observed in the diffuse medium by:\\
L.-Å. Nyman,\\
**[[https://ui.adsabs.harvard.edu/abs/1984A%26A...141..323N|Detection of CS and C<sub>2</sub>H in Absorption]]**,\\
//Astron. Astrophys.// **141** 323–327 (1984).

C<sub>2</sub>D was detected near 216.4 GHz with the 5 m MWO telescope toward Orion KL by:\\
F. Combes, F. Boulanger, P. J. Encrenaz, M. Gerin, M. Bogey, C. Demuynck, J. L. Destomb,\\
**[[https://ui.adsabs.harvard.edu/abs/1985A%26A...147L..25C|Detection of Interstellar CCD]]**,\\
//Astron. Astrophys.// **147** L25–L26 (1985).

The detection of the <sup>13</sup>C isotopic species was reported in:\\
A. H. Saleck, R. Simon, G. Winnewisser, J. G. A. Wouterloot,\\
**[[https://doi.org/10.1139/p94-098|Detection of Interstellar <sup>13</sup>CCH and C<sup>13</sup>CH]]**,\\
//Can. J. Phys.// **72** 747–754 (1994).

Three out of four observable fine structure components of the //N// = 3 – 2 rotational transition of vibrationally excited C<sub>2</sub>H were reportedly not overlapped:\\
E. D. Tenenbaum, J. L. Dodd, S. N. Milam, N. J. Woolf, and L. M. Ziurys,\\
**[[https://doi.org/10.1088/2041-8205/720/1/L102|Comparative Spectra of Oxygen-rich Versus Carbon-rich Circumstellar Shells: VY Canis Majoris and IRC +10216 at 215-285 GHz]]**,\\
//Astrophys. J.// **720** L102–L107 (2010).\\
The features were detected in the outer part of the circumstellar envelope of CW Leo; hence, the detection must be facilitated by formation pumping or by IR pumping.

The //A// <sup>2</sup>Π – //X// <sup>2</sup>Σ<sup>+</sup> first excited electronic transition of C<sub>2</sub>H is distributed over at least 5 bands because of vibronic coupling of the //A// with various higher excited vibrational Π states of the ground electronic //X// state. Almost 30 lines belonging to two of the stronger bands were detected with the Mayall telescope near 4011 and 4108 cm<sup>–1</sup> in the outer region of the circumstellar envelope of CW Leo by\\
J. J. Keady and K. H. Hinkle,\\
**[[https://ui.adsabs.harvard.edu/abs/1988ApJ...331..539K|C<sub>2</sub>H in the 2 micron Infrared Spectrum of IRC +10216]]**,\\
//Astrophys. J.// **331** 539–546 (1988).\\
The rotational temperature of 12.5 ± 1.5 K suggests a peak abundance about 1200 stellar radii away from CW Leo, and the line intensities indicate a high fractional abundance with respect to H<sub>2</sub>.


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Contributor(s): H. S. P. Müller; 1, 2012

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