1998A&A...337..275N

We have searched for millimetre-wave line emission from ethylene oxide (c-C₂H₄O) and its structural isomer acetaldehyde (CH₃CHO) in 11 molecular clouds using SEST. Ethylene oxide and acetaldehyde were detected through multiple lines in the hot cores NGC 6334F, G327.3-0.6, G31.41+0.31, and G34.3+0.2. Acetaldehyde was also detected towards G10.47+0.03, G322.2+0.6, and Orion 3'N, and one ethylene oxide line was tentatively detected in G10.47+0.03. Column densities and rotational excitation temperatures were derived using a procedure which fits the observed line intensities by finding the minimum χ²-value. The resulting rotational excitation temperatures of ethylene oxide and acetaldehyde are in the range 16-38K, indicating that these species are excited in the outer, cooler parts of the hot cores or that the excitation is significantly subthermal. For an assumed source size of 20'', the deduced column densities are (0.6-1)x10¹⁴cm^–2 for ethylene oxide and (2-5)x10¹⁴cm^–2 for acetaldehyde. The fractional abundances with respect to H₂ are X[c-C₂H₄O]=(2-6)x10^–10, and X[CH₃CHO]=(0.8-3)x10^–9. The ratio X[CH₃CHO]/X[c-C₂H₄O] varies between 2.6 (NGC 6334F) and 8.5 (G327.3-0.6). We also detected and analysed multiple transitions of CH₃OH, CH₃OCH₃, C₂H₅OH, and HCOOH. The chemical, and possibly evolutionary, states of NGC 6334F, G327.3-0.6, G31.41+0.31, and G34.3+0.2 seem to be very similar.