Vibrationally excited CH3CN and HC3N in Orion

The detection of vibrationally excited CH3CN (methyl cyanide) in the nu8 vibrational state, which lies approximately 520 K above the ground state, is reported. The line velocities (6 km/s) and widths (10 km/s) indicate that this emission is being produced in the hot core region located in the central portion of the Orion molecular cloud. The vibrational excitation temperature is estimated to be between 200 and 350 K, which could be produced by infrared radiation within a region of about 3 arcsec of an L = 100,000 solar-luminosity IR source. The density required for collisional excitation seems excessive for the minimum source size which is also approximately 3 arcsec. Detection of the J = 12-11 rotational transition of HC3N in the 2nu7 state confirms the degree of vibrational excitation determined previously (Goldsmith et al, 1982); the HC3N excitation is somewhat less than that of CH3CN and the source size is about 6 arcsec in diameter.