L. E. Snyder, D. Buhl, B. Zuckerman, and P. Palmer
reported on the
Microwave Detection of Interstellar Formaldehyde
Phys. Rev. Lett. 22, 679–681 (1969).
The 1_1,0 – 1_1,1 transition near 4830 MHz was detected in absorption with the 43 m NRAO telescope toward numerous continuum sources: M17, W3, W3(OH), W49, NGC 2024, DR 21, W43, W44, W51, Sgr A, Sgr B2, W33, NGC 6334, Cas A, and 3C 123. Formaldehyde was only the third poly-atomic molecule detected in space, the fourth molecule by radio astronomy, and the seventh molecule in space.

P. Palmer, B. Zuckerman, D. Buhl, and L. E. Snyder
studied
Formaldehyde Absorption in Dark Nebulae
Astrophys. J. 156, L147–L150 (1969).
The same transition was detected in several positions of Heiles' Cloud 2 with ¹H hyperfine structure resolved in part.

A. Heithausen, U. Mebold, and H. W. de Vries
performed
A Survey of Formaldehyde in High Galactic Latitudes
Astron. Astrophys. 179, 263–267 (1987),
with the Effelsberg 100 m telescope which led to the detection of H₂CO in translucent clouds.

A. G. Nash
studied
The abundance ratio of formaldehyde to ammonia in molecular clouds observed toward radio continuum sources
Astrophys. J. Suppl. Ser. 72, 303–322 (1990),
which led to the detection of H₂CO in diffuse clouds. The Effelsberg 100 m telescope and the 43 m NRAO dish were employed.

J. Cernicharo, M. Guélin, J. Penalver, J. Martín-Pintado, and R. Mauersberger
identified
A 200 km/s molecular outflow in the protoplanetary nebula CRL 618
Astron. Astrophys. 222, L1–L4 (1989)
in the course of a 3 and 1.3 mm line survey using the IRAM 30 m telescope. The C-rich PPN is also known as V353 Aur.

M. Lindqvist, H. Olofsson, A. Winnberg, and L.-Å. Nyman
investigated
Carbon-bearing molecules in the envelopes around oxygen-rich stars – First detection of formaldehyde in an oxygen-rich circumstellar envelope
Astron. Astrophys. 263, 183–189 (1992).
The O-rich CSE was that of the PPN QX Pup, also known as OH231.8+4.2.

K. E. S. Ford, D. A. Neufeld, P. Schilke, and G. J. Melnick
reported on the
Detection of Formaldehyde toward the Extreme Carbon Star IRC+10216
Astrophys. J. 614, 990–1006 (2004).
The source, also known as CW Leo, is an asymptotic giant branch (AGB) star, which is the evolutionary phase prior to the PPN and PN phases.

L. Velilla Prieto, C. Sánchez Contreras, J. Cernicharo, M. Agúndez, G. Quintana-Lacaci, V. Bujarrabal, J. Alcolea, C. Balanca, F. Herpin, K. M. Menten, and F. Wyrowski
mentioned the detection of H₂CO in the envelope of an O-rich AGB star in
The millimeter IRAM-30 m line survey toward IK Tauri
Astron. Astrophys. 597, Art. No. A25 (2017).

Maser activity in formaldehyde was found by
D. Downes, T. L. Wilson
in their work on
Formaldehyde Line Emission at 4.8 GHz near NGC 7538
Astrophys. J. 614, L77–L78 (1974)
employing the Effelsberg 100 m telescope.

B. Zuckerman, P. Palmer, L. E. Snyder, and D. Buhl
reported on the
Detection of Interstellar H₂¹³C¹⁶O
Astrophys. J. 157, L167–L171 (1969)
in Sgr B2, Sgr A, and possibly W51 using the NRAO 43 m telecope.

F. F. Gardner, J. C. Ribes, and B. F. C. Cooper
reported on the
Detection of the ¹⁸O Isotop(ologu)e of Formaldehyde at 4388 MHz
Astrophys. Lett. 157, 181–183 (1971)
in Sgr B2 using the Parkes 64 m telescope.

W. D. Langer, M. A. Frerking, R. A. Linke, and R. W. Wilson, reported on the
Detection of Deuterated Formaldehyde in Interstellar Clouds
Astrophys. J. 157, L167–L171 (1969).
The Bell Lab. 7 m telescope was used to detect the 2_0,2 – 1_0,1 transition near 128813 MHz in emission toward the dark clouds L134 N and Heiles' Cloud 2.

B. E. Turner
reported on the
Detection of doubly deuterated interstellar formaldehyde (D₂CO) – an indicator of active grain surface chemistry
Astrophys. J. 362, L29–L33 (1990).
Four transition were detected in emission in the Compact Ridge of Orion KL using the NRAO 12 m telescope.

M. V. Persson, J. K. Jørgensen, H. S. P. Müller, A. Coutens, E. F. van Dishoeck, V. Taquet, H. Calcutt, M. H. D. van der Wiel, T. L. Bourke, and S. F. Wampfler
reported on the
The ALMA-PILS Survey: Formaldehyde Deuteration in Warm Gas on Small Scales toward IRAS 16293–2422 B
Astron. Astrophys. 610, Art. No. A54 (2018).
They identified unambiguously H₂C¹⁷O and D₂¹³CO; lines of HD¹³CO were too weak in that frequency range to be detected.

Contributor(s): H. S. P. Müller; 01, 2017; 03, 2018