===== On the Detection of Butadiynyl, C4H, in Space =====

The butadiynyl radical in its <sup>2</sup>Σ ground vibrational state has been detected between 85 and 115 GHz with the NRAO 11 m telescope in the circumstellar envelope of the carbon-rich late-type star CW Leo, also known as IRC +10216, in eight fine structure lines of four rotational transitions (//N// = 9 – 8 to 12 – 11) by\\
M. Guélin, S. Green, and P. Thaddeus,\\
**[[https://ui.adsabs.harvard.edu/abs/1978ApJ...224L..27G|Detection of the C<sub>4</sub>H Radical toward IRC +10216]]**,\\
//Astrophys. J.// **224** L27–L30 (1978)\\
before it was actually detected in the laboratory. Two rotational transitions had been reported previously as U-lines in two different publications. The highest frequency fine structure component seems to be overlapped.

Soon thereafter, it was detected with the OSO 20 m telescope toward TMC-1 by:\\
W. M. Irvine, B. Höglund, P. Friberg, J. Askne, and J. Elldér,\\
**[[https://ui.adsabs.harvard.edu/abs/1981ApJ...248L.113I|The Increasing Chemical Complexity of the Taurus Dark Clouds – Detection of CH<sub>3</sub>CCH and C<sub>4</sub>H]]**,\\
//Astrophys. J.// **248** L113–L117 (1981).\\
Four hyperfine components of the //N// = 3 – 2 transition were detected near 28.55 GHz.

The radical was also observed in the diffuse medium by:\\
M. B. Bell, P. A. Friedman, and H. E. Matthews,\\
**[[https://ui.adsabs.harvard.edu/abs/1983ApJ...273L..35B|The Detection of Butadiynyl (C<sub>4</sub>H) in Absorption against Cassiopeia A]]**,\\
//Astrophys. J.// **273** L35–L39 (1983).

Moreover, the molecule was also detected in several PDRs, namely the Horsehead nebula (B 33), IC 63, and in the //ρ// Oph L1688-W cloud interface:\\
D. Teyssier, D. Fossé M. Gerin, J. Pety, A. Abergel, and E. Roueff,\\
**[[https://doi.org/10.1051/0004-6361:20034534|Carbon Budget and Carbon Chemistry in Photon Dominated Regions]]**,\\
//Astron. Astrophys.// **417** 135–149 (2004).

Vibrationally excited C<sub>4</sub>H was also detected:\\
M. Guélin, J. Cernicharo, S. Navarro, D. R. Woodward, C. A. Gottlieb, and P. Thaddeus,\\
**[[https://ui.adsabs.harvard.edu/abs/1987A%26A...182L..37G|New doublets in IRC +10216 – Vibrationally Excited C<sub>4</sub>H?]]**,\\
//Astron. Astrophys.// **182** L37–L39 (1987).\\
Subsequent laboratory work revealed these transitions to be due to the //v//<sub>7</sub> = 1 Π state and the //v//<sub>7</sub> = 2 Σ and Δ states:\\
S. Yamamoto, S. Saito, M. Guélin, J. Cernicharo, H. Suzuki, and M. Ohishi,\\
**[[https://ui.adsabs.harvard.edu/abs/1987ApJ...323L.149Y|Laboratory Microwave Spectroscopy of the Vibrational Satellites for the //ν//<sub>7</sub> and 2//ν//<sub>7</sub> states of C<sub>4</sub>H and their Astronomical Identification]]**,\\
//Astrophys. J.// **323** L149–L153 (1987).\\
Unfortunately, no laboratory data is available for higher excited vibrational states !

C<sub>4</sub>D was detected toward TMC-1 by:\\
B. E. Turner,\\
**[[https://ui.adsabs.harvard.edu/abs/1989ApJ...347L..39T|Detection of C<sub>4</sub>D – Implications for Ion-Molecule Chemistry]]**,\\
//Astrophys. J.// **347** L39–L42 (1989).

The detection of the <sup>13</sup>C isotopic species was reported in:\\
J. Cernicharo, M. Guélin, and C. Kahane,\\
**[[https://ui.adsabs.harvard.edu/abs/2000A%26AS..142..181C|A λ 2 mm Molecular Line Survey of the C-star Envelope IRC+10216]]**,\\
//Astron. Astrophys. Suppl. Ser.// **142** 181–215 (2000).


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Contributor(s): H. S. P. Müller; 5, 2007

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