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).
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.