Detection of Methyl Formate in the Second Torsionally Excited State (vt = 2) in Orion KL
Abstract
We report on the detection of rotational transitions of methyl formate (HCOOCH3) in the second torsionally excited state (υt = 2) of the methyl internal rotation in Orion Kleinmann-Low (KL). Recent progress in the microwave spectroscopy of methyl formate in laboratories and sensitive radioastronomical observations have made it possible to assign ∼ 20 emission lines to methyl formate in the υt = 2 state. The beam-averaged column density obtained with the Nobeyama 45 m radiotelescope is (3.0±1.5) × 1014 cm-2 in this state. In addition to this state, we also observed lines in the ground and the first torsionally excited states (υt = 0 and 1) in Orion KL. The obtained rotational temperatures for the υt = 0 and 1 states are 43±19 and 53±18 K, respectively. The column densities for the υt = 0 and 1 states are (3.8±11.2) × 1015 and (9.8±12.3) × 1014 cm-2, respectively, which are 13 and 3 times the column density in the υt = 2 state. Data of these three states can be explained by a single vibrational temperature of 124±15 K, which is significantly higher than the rotational temperatures. As excitation mechanisms, there are two possibilities: collision with H2 and pumping by far-infrared radiation from star-formation activities. Considering the temperature difference and Einstein's A coefficients of the torsional states, we cannot rule out either of these possibilities. This detection further indicates that many more unidentified lines in Orion KL will be due to low-lying excited states of methyl formate and other organic molecules.
- Publication:
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Publications of the Astronomical Society of Japan
- Pub Date:
- August 2012
- DOI:
- 10.1093/pasj/64.4.89
- Bibcode:
- 2012PASJ...64...89T
- Keywords:
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- ISM: individual(Orion KL);
- ISM: molecules;
- line: identification;
- radio lines: ISM