The ethynyl radical in its 2Σ 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,
The Ethynyl Radical C2H A New Interstellar Molecule,
Astrophys. J. 193 L115L119 (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
Detection of C2H in Cold Dark Clouds,
Astrophys. J. 239 944854 (1980).
The radical was also observed in the diffuse medium by:
Detection of CS and C2H in Absorption,
Astron. Astrophys. 141 323327 (1984).
C2D 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,
Detection of Interstellar CCD,
Astron. Astrophys. 147 L25L26 (1985).
The detection of the 13C isotopic species was reported in:
A. H. Saleck, R. Simon, G. Winnewisser, J. G. A. Wouterloot,
Detection of Interstellar 13CCH and C13CH,
Can. J. Phys. 72 747754 (1994).
Three out of four observable fine structure components of the
N = 3 2 rotational transition of
vibrationally excited C2H were reportedly
E. D. Tenenbaum, J. L. Dodd, S. N. Milam, N. J. Woolf, and L. M. Ziurys,
Comparative Spectra of Oxygen-rich Versus Carbon-rich Circumstellar Shells: VY Canis Majoris and IRC +10216 at 215-285 GHz,
Astrophys. J. 720 L102L107 (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 2Π X 2Σ+
first excited electronic transition of C2H 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 cm1
in the outer region of the circumstellar envelope of CW Leo
J. J. Keady and K. H. Hinkle,
C2H in the 2 micron Infrared Spectrum of IRC +10216,
Astrophys. J. 331 539546 (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 H2.