HCO is frequently discussed in conjunction with PDRs (photo dissiciation regions). L. E. Snyder, J. M. Hollis, and B. L. Ulich, reported on the
Radio Detection of the Interstellar Formyl Radical
Astrophys. J. 208, L91–L94 (1976).
The NRAO 11 M dish was used to detect the strongest HFS components (J = 1.5 – 0.5, F = 2 – 1) of the 1₀₁ – 0₀₀ transition near 86.671 GHz in emission toward sources showing carbon recombination lines: W3, W51, NGC 2024C, and K3-50. Please note that the NGC 2024 position was stated more precisely in the follwing paragraph.

J. M. Hollis and E. Churchwell
looked at the
Comparison of C⁺ Distributions with new Interstellar Sources of HCO Emission
Astrophys. J. 271, 170–744 (1983).
The same HFS component was observed with the same instruments as above. New detections were made at a position close to Sgr B2(OH) and in DR21C.

L. E. Snyder, M. S. Schenewerk, and J. M. Hollis,
reported on the
Observations of Several new Transitions of Interstellar HCO
Astrophys. J. 298, 360–368 (1985).
Observations of the 2₀₂ – 1₀₁ and 3₀₃ – 2₀₁ transitions were made. In addition,
Å Hjalmarson, M. S. Schenewerk, and L. E. Snyder,
reported on
Interstellar HCO – Detection of the Missing 3 Millimeter Quartet
Astrophys. J. 298, L71–L74 (1986).

H. S. Liszt, J. Pety, M. Gerin, and R. Lucas,
detected with the IRAM 30 m telescope
HCO, //c//-C₃H and CF⁺: three new molecules in diffuse, translucent and “spiral-arm” clouds
Astron. Astrophys. 564, Art. No. A64 (2014).

The formyl radical is also found quite commonly toward certain types of dark clouds, as found, e.g., by
M. Agúndez, J. Cernicharo, and M. Guélin
in the report on the
Discovery of Interstellar Ketenyl (HCCO), a Surprisingly Abundant Radical
Astron. Astrophys. 577, Art. No. L5 (2015).

Contributor(s): H. S. P. Müller; 05, 2015