High-temperature methyl cyanide in Orion molecular cloud 1

The K = 0 through K = 6 lines of the J = 13-12 and J = 12-11 rotational transitions of methyl cyanide at 239 and 221 GHz are observed toward OMC-1. The relative strengths of the transitions are used to obtain the excitation temperatures of the CH3CN emitting region, and the easily excited K = 0-2 transitions are found to yield 80-12 K, while the K = 4-6 transitions yield 270 K. The low temperature region is characterized by a small CH3CN line width with a velocity dispersion of 5 km/s, and the hot region produces a larger velocity dispersion of 8 km/s. The use of CH3CN as a temperature probe allows the simultaneous observation of transitions of significantly different energy in a single spectral calibration, thus eliminating uncertainties of telescope calibration, sideband gain, or relative pointing in the temperature determination.