Formic acid, HC(O)OH, seems to be a fairly typical hot core molecule
that is quite abundant. The 111 – 110
transition of this molecule was the first one to be detected
– toward the massive star-forming region Sgr B2 near the Galactic
center:
B. Zuckerman, J. A. Ball, and C. A. Gottlieb,
Microwave Detection of Interstellar Formic Acid
Astrophys. J. 163, L41–L45 (1971);
the detection of a second transition was reported by
G. Winnewisser and E. Churchwell,
Detection of Formic Acid in Sagittarius B2 by its
211 – 212 Transition
Astrophys. J. 200, L33–L36 (1975).
However, the detection of formic acid in a dark cloud was also reported:
W. M. Irvine, P. Friberg, N. Kaifu, H. E. Matthews, Y. C. Minh,
M. Ohishi, and S. Ishikawa,
Detection of Formic Acid in the Cold, Dark Cloud
L134N
Astron. Astrophys. 229, L9–L12 (1990).
The molecule has also been detected toward a low-mass
star-forming region:
S. Cazaux, A. G. G. M. Tielens, C. Ceccarelli, A. Castets, V. Wakelam,
E. Caux, B. Parise, D. Teyssier,
The Hot Core around the Low-mass Protostar IRAS 16293-2422:
Scoundrels Rule!
Astrophys. J. 593, L51–L55 (2003).
The number of observed transitions for isotopic species is too small to be conclusive.