Detection of Methyl Formate in the Second Torsionally Excited State (v(t)=2) in Orion KL

作者：Takano Shuro^*; Sakai Yusuke; Kakimoto Suguru; Sasaki Mutsumi; Kobayashi Kaori

来源：Publications of the Astronomical Society of Japan, 2012, 64(4): 89.

DOI：10.1093/pasj/64.4.89

• ISM: individual(Orion KL)
• ISM: molecules
• line: identification
• radio lines: ISM

We report on the detection of rotational transitions of methyl formate (HCOOCH3) in the second torsionally excited state (v(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 similar to 20 emission lines to methyl formate in the v(t) = 2 state. The beam-averaged column density obtained with the Nobeyama 45 m radiotelescope is (3.0 +/- 1.5) x 10(14) cm(-2) in this state. In addition to this state, we also observed lines in the ground and the first torsionally excited states (v(t) = 0 and 1) in Orion KL. The obtained rotational temperatures for the v(t) = 0 and 1 states are 43 +/- 9 and 53 +/- 8 K, respectively. The column densities for the v(t) = 0 and 1 states are (3.8 +/- 1.2) x 10(15) and (9.8 +/- 2.3) x 10(14) cm(-2), respectively, which are 13 and 3 times the column density in the v(t) = 2 state. Data of these three states can be explained by a single vibrational temperature of 124 +/- 5 K, which is significantly higher than the rotational temperatures. As excitation mechanisms, there are two possibilities: collision with H-2 and pumping by far-infrared radiation from star-formation activities. Considering the temperature difference and Einstein%26apos;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.

• 出版日期2012-8-25