Formic Acid in the ISM

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).

Therefore, the detection of the molecule toward a low-mass proto-star is not so surprising:
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 same holds for the detection of the molecule in several Galactic center molecular clouds which have low dust temperatures (10–20 K), even lower rotational temperatures (around 10 K), which are moderately dense (a few tenthousand molecules per cubic centimeter), but which are kinetically moderately warm (about 100 K or more). Two transitions were detected with the IRAM 30 m telescope in the 3 mm range by
M. A. Requena-Torres, J. Martín-Pintado, A. Rodríguez-Franco, S. Martín, N. J. Rodríguez-Fernández, and P. de Vicente,
Organic Molecules in the Galactic Center – Hot Core Chemistry without Hot Cores
Astron. Astrophys. 455, 971–985 (2006).

S. Cuadrado, J. R. Goicoechea, O. Roncero, A. Aguado, B. Tercero, and J. Cernicharo
reported on the
Trans-cis Molecular Photoswitching in Interstellar Space
Astron. Astrophys. 596, Art. No. L1 (2016).
The authors used the IRAM 30 m telescope in the 3 mm region to search for trans-HCOOH as well as cis-HCOOH at different positions of the Orion Bar PDR. The cis-HCOOH conformer is about 1960 K higher in energy than the trans conformer, and the barrier to coversion from trans to cis is about 7000 K. The trans conformer was found to be widespread whereas the first detection of the cis conformer conformer was limited to the interface region in which the densities are already quite high, but still enough UV radiation penetrates the ISM. There are indications that the conversion is mediated by UV radiation via an electronically excited state of formic acid.

The number of observed transitions for isotopic species is still too small to be conclusive.

Contributor(s): H. S. P. Müller; 2002; 02, 2007; 05, 2009; 07, 2012; 11, 2016