The Detection and Mapping Observations of C 2H 5OH in Orion Kleinmann-Low

Four lines of C2H5OH were detected in Orion KL, using the 45 m radio telescope of Nobeyama Radio Observatory, and mapping observations were made of two of these transitions. The distribution of C2H5OH peaks about 10" south of IRc2, and the peak position is very close to that of CH3OH, HCOOCH3, and (CH3)2O. The peak column density of C2H5OH in Orion KL is estimated to be ∼1 × 1015 cm-2, and the excitation temperature is calculated to be ∼70 K. This column density corresponds to a fractional abundance relative to H2 of 3 × 10-9. This value is larger by more than two orders of magnitude than the calculated abundance of C2H5OH from models utilizing gas-phase reactions. The distribution and the high abundance of C2H5OH in Orion KL suggests that the molecule is formed in grain mantles and then evaporated. The excitation of C2H5OH is well explained by LTE, and this implies that the expected brightness temperatures of a-type transitions of C2H5OH are quite weak, contrary to the previous report by Turner (1991).