Vibrationally excited CH3CN and HC3N in Orion

Abstract

We report the detection of vibrationally excited CH 3 CN (methyl cyanide) in the v 8 vibrational state, which lies approximately 520 K above the ground state. The line velocities (~6 km s-1) and widths (~ 10 km s~ x) 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 ~3" (2 x 10 16 cm) of an L = 10 5 L 0 IR source. The density required for collisional excitation, lO^-lO 11 cm-3 , seems excessive for the minimum source size which is also approximately 3". Detection of the J = 12-► 11 rotational transition of HC 3 N (cyanoacetylene) in the 2v 7 state confirms the degree of vibrational excitation determined previously (Goldsmith et al); the HC 3 N excitation (7^ x ~ 150 K) is somewhat less than that of CH 3 CN and the source size is ~6" in diameter. S0 2 emission in the v 2 vibrationally excited state was searched for, but not detected.