The HCO radical was detected in the ISM a long time ago. The detection of the HCCO radical, however, was found only fairly recently.
B. A. McGuire, A. M. Burkhardt, C. N. Shingledecker, S. V. Kalenskii, E. Herbst, A. J. Remijan, and M. C. McCarthy
reported on the
Detection of Interstellar HC<sub>5</sub>O in TMC-1 with the Green Bank Telescope
Astrophys. J. 843, Art. No. L28 (2017).
Two Λ-components of butadiynylformyl, each consisting of two HFS components, of the J = 17/2 – 15/2 transition were detected in emission near 21.945 GHz.
In addition, the hexatriynylformyl radical, HC7O, was tentatively detected through stacking the Λ-components of two transitions, J = 35/2 – 33/2 and 37/2 – 35/2 near 19.204 and 20.302 GHz, respectively. These reports may appear as space oddities, but we shall know more eventually.

M. A. Cordiner, S. B. Charnley, Z. Kisiel, B. A. McGuire, and Y.-J. Kuan
reported on the
Deep K-band Observations of TMC-1 with the Green Bank Telescope: Detection of HC<sub>7</sub>O, Nondetection of HC<sub>11</sub>N, and a Search for New Organic Molecules
Astrophys. J. 850, Art. No. 187 (2017).
The Λ-components of two hexatriynylformyl transitions, J = 35/2 – 33/2 near 19.204 GHz (as before) and 33/2 – 31/2 near 18.107 GHz were detected weakly. Stacking of these data together with those from above yielded a reasonable detection.
See also HCCCO contribution.

J. Cernicharo, M. Agúndez, C. Cabezas, B. Tercero, N. Marcelino, R. Fuentetaja, J. R. Pardo, and P. de Vicente
reported on the
Discovery of HCCCO and C<sub>5</sub>O in TMC-1 with the QUIJOTE Line Survey
Astron. Astrophys. 656, Art. No. L21 (2021).
Propynonyl, HC3O, was identified through two rotational transitions with 2 ≤ J ≤ 4 in the course of a molecular line survey of the prototypical cold dark molecular cloud TMC-1 carried out with the Yebes 40 m radio telescope between 31.0 and 50.3 GHz. 7 of the 8 strong hyperfine components of four fine structure components were detected clearly; two HFS components were unresolved. A value of Trot = 7 K was taken from C3O. HC5O is the most abundant of the HCnO molecules with n ≥ 1. Setting its relative abundance to 100, they obtained for HCO, HC2O, HC3O, HC5O, and HC7O the approximate ratios 79 : 55 : 9 : 100 : 46. Only upper limits were derived for HC4O and HC6O; these were in both cases similar to those of HC3O and are thus somewhat constraining.

Contributor(s): H. S. P. Müller; 07, 2017; 12, 2017; 12, 2021

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