Ethanal, CH3CHO, also known as acetaldehyde, was among
the early molecules to be detected in space. Its first detection
was reported in a conference proceeding:
C. A. Gottlieb,
Detection of Acetaldehyde in Sagittarius
Molecules in the Galactic Environment eds. M. A. Gordon and L. E. Snyder, Wiley-Interscience, New York, 181186 (1973).
The a-type Q-branch transition 110 111 near 1065 MHz was detected toward Sagittarius A and Sgr B2.
Soon thereafter, the observation of the
211 212 transition near
3195 MHz toward Sgr B2 with the Parkes 64 m telescope
N. Fourikis, M. W. Sinclair, B. J. Robinson, P. D. Godfrey, and R. D. Brown,
Microwave Emission of the 211 → 212 Rotational Transition in Interstellar Acetaldehyde
Aust. J. Phys. 27, 425430 (1974).
Many more transitions were detected toward hot cores such as Sgr B2 or
OMC-1. However, the rotational temperatures were usually quite low
(50 K or even considerably lower) and suggest an extended distribution.
A fairly recent mapping of the 110 111
transition toward Sgr B2 with the GRMT demonstrates this rather nicely:
J. N. Chengalur and N. Kanekar,
Widespread Acetaldehyde near the Galactic Centre
Astron. Astrophys. 403, L43L46 (2003).
Ethanal has also been detected in cold clouds:
H. E. Matthews, P. Friber, and W. M. Irvine,
The Detection of Acetaldehyde in Cold Dust Clouds
Astrophys. J. 290, 609614 (1985).
Both A and E internal rotation components of the 101 000 transition near 19264 MHz were detected with the NRAO 140-foot telescope toward TMC-1 and L134N.
It was also found in one of three translucent clouds (CB 17):
B. E. Turner, R. Terzieva, and E. Herbst,
The Physics and Chemistry of Small Translucent Molecular Clouds. XII. More Complex Species Explainable by Gas-Phase Processes
Astrophys. J. 518, 699732 (1999).
V. Thiel, A. Belloche, K. M. Menten, R. T. Garrod, and H. S. P. Müller,
Complex Organic Molecules in Diffuse Clouds along the Line of Sight to Sgr B2
Astron. Astrophys. 605 (2017), Art. No. L6.
Three low-J transitions of ethanal were detected with ALMA between 84.2 and 101.4 GHz in absorption in two velocity components corresponding to Galactic center diffuse clouds. The absorption was detected toward the ultracompact H II region K4 in the Sagittarius B2(N) complex.
A. Belloche, H. S. P. Müller, K. M. Menten, P. Schilke, and C. Comito
Complex Organic Molecules in the Interstellar Medium: IRAM 30 m Line Survey of Sagittarius B2(N) and (M)
Astron. Astrophys. 559, Art. No. A47 (2013).
They observed several transitions in the first excited torsional state. The vibrational temperature of about 150 K is higher than the rotational temperature of about 100 K. The difference was viewed as an indication of (far-) infrared pumping.