Detection of methyl formate in the second torsionally excited state (ν_t = 2) in Orion KL

We report on the detection of rotational transitions of methyl formate (HCOOCH₃) in the second torsionally excited state (ν_t = 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 ν_t = 2 state. The beam-averaged column density obtained with the Nobeyama 45m 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 (ν_t = 0 and 1) in Orion KL. The obtained rotational temperatures for the ν_t = 0 and 1 states are 43 ± 9 and 53 ± 8K, respectively. The column densities for the ν_t = 0 and 1 states are (3.8 ± 1.2) × 10¹⁵ and (9.8 ± 2.3) × 10¹⁴ cm^-2, respectively, which are 13 and 3 times the column density in the ν_t = 2 state. Data of these three states can be explained by a single vibrational temperature of 124 ± 5K, 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.